#strings

181 strings interview questions, asked at Meta, Amazon, Google and 26 more companies. Last asked Oct 2026.

Review a CSV-building function for correctness and performance bugsCodingYou are shown a short function (in Java or Python) that turns a table, a list of rows of string cells, into CSV text by appending to a string in a loop. Say what is wrong with its…
Last asked Oct 2026
Merchant onboarding CSV validatorCodingMerchants submit their business details as rows of a simple CSV. Before a merchant can accept payments, every row must pass a set of checks. Write validaterecords(data…
Last asked Oct 2026
Concatenated wordsCodingGiven a list of distinct words, return every word that can be split into two or more pieces that are each a word in the list (the same word may be used more than once). A shorter…
Last asked Oct 2026
+3
Basic calculator IICodingEvaluate a string expression of non-negative integers with +, -, , / and spaces, no parentheses, respecting precedence (integer division truncates toward zero). Variants reported:…
Last asked Sep 2026
Minimum deletions to balance a parentheses stringCodingA string contains only ( and ). Delete as few characters as possible so the rest is balanced, and return the resulting string.
Last asked Sep 2026
+2
Encode and decode strings with length prefixes and count codesCodingSerialize a list of arbitrary strings into one string and back without ambiguity. A second variant decodes a compact code where some letters are written as two digits with a…
Last asked Sep 2026
+1
Decode stringCodingA string is encoded with the rule ktext, meaning text repeated k times; k is a positive integer that may have several digits, encoded parts can be nested, and plain letters can…
Last asked Sep 2026
String compressionCodingGiven a non-empty list of single characters, compress it with run-length encoding: for each run of equal characters write the character, followed by the run's length when it is…
Last asked Sep 2026
Delete one '5' from a digit string to get the largest numberCodingYou get a string of decimal digits that contains at least one '5'. Remove exactly one of the '5' characters so that the number left is as large as possible, and return that…
Last asked Sep 2026
Valid word abbreviationCodingA word can be abbreviated by replacing non-overlapping, non-adjacent substrings with their lengths, so internationalization may become i18n or i5a11o1. Given a word and an…
Last asked Sep 2026
Text box formatterCodingImplement formatboxes(sentences: liststr) - liststr. Each sentence is a non-empty string of printable ASCII characters and spaces. Draw every sentence in its own ASCII box and…
Last asked Sep 2026
+2
Greedy longest-match tokenizerCodingImplement a greedy longest-match tokenizer. You are given a string text and a list dictionary. Each entry has the form "<key:<id". key is a non-empty token string and id is its…
Last asked Sep 2026
+1
Resolve a cd command to an absolute pathCodingWrite a function that takes the current directory and the argument to cd, and returns the directory pwd would print afterwards. Handle ., .., repeated and trailing slashes, and…
Last asked Sep 2026
First position of any anagram of a pattern in a stringCodingReturn the smallest index at which some rearrangement of a query string appears as a contiguous substring of a text, or -1. Keep a running character count over a fixed-size window…
Last asked Sep 2026
Parse a custom record format with mapped fieldsCodingParse records written in a custom text format: a few fixed comma-separated fields first (for example a machine code, a timestamp and two coded fields), then a free-form section of…
Last asked Sep 2026
Return every word that starts with a prefixCodingBuild a structure over a word list, once, that answers many prefix queries: for a prefix, return every word in the list that starts with it, without rescanning the whole list. A…
Last asked Sep 2026
Character at an index of a run-length encoded stringCodingA string is stored run-length encoded, like "B1A2E3C1" for BAAEEEC. Return the character at a given index without decoding the string. Follow-up: many queries; precompute prefix…
Last asked Sep 2026
Does a seven-segment display read the same rotated 180 degrees?CodingAn LED display shows a row of digits on seven-segment cells. Decide whether the display reads exactly the same after rotating it by 180 degrees. Work out which digits map to which…
Last asked Sep 2026
+1
Custom sort stringCodingGiven an order string of distinct letters and a string s, rearrange s so its letters follow the order given; letters missing from order may go anywhere. Count letters and emit…
Last asked Sep 2026
Merge two strings on their longest overlapCodingGiven two strings, join them so that the longest overlap between the end of one and the start of the other appears only once. Try both orders (first then second, and second then…
Last asked Sep 2026
Most frequent substring under length and distinct-letter limitsCodingGiven a string, consider only substrings whose length lies between a minimum and a maximum and that contain at most a given number of distinct letters. Return how many times the…
Last asked Sep 2026
Expand placeholders from a variable storeCodingImplement a small store of variables, like the export NAME="Tom" lines of a shell startup file. Any string can refer to a variable as %NAME%; expanding the string replaces each…
Last asked Sep 2026
Longest repeating character replacementCodingGiven a string of uppercase letters and an integer k, you may change at most k characters to any other letter. Return the length of the longest substring that can be made of one…
Last asked Sep 2026
Highlight categorized phrases in a reviewCodingReviews on a travel site should show key phrases with their category. Write a function that marks every mapped phrase in a review. mapping is a list of (phrase, category) pairs…
Last asked Sep 2026
Predict the most frequent next word in constant timeCodingFrom a body of text, build a predictor that, given a word, returns the word that most often came right after it. Each prediction must take O(1) time. Discuss how you keep that…
Last asked Sep 2026
Parse a signed integer in any base, with overflow checksCodingWrite a production-quality parser that turns a string into a 32-bit signed integer in a given base. Handle signs, digits and letters valid for the base, and report invalid syntax…
Last asked Aug 2026
Reconstruct digits from scrambled English number wordsCodingA string holds the shuffled letters of some English digit words (zero to nine). Recover the digits and return them in ascending order, repeats included. Explain the order in which…
Last asked Aug 2026
Convert a hexadecimal address to dotted IPv4CodingGiven a 32-bit address written in hexadecimal, return it in dotted-decimal IPv4 form, for example C0A80001 becomes 192.168.0.1. Decide and state how you treat an optional 0x…
Last asked Aug 2026
Stable, unique column names across schema snapshotsCodingYou receive successive snapshots of table schemas and must give every column an effective name. Dots in an original name become underscores, and effective names must be unique…
Last asked Aug 2026
Valid palindrome ignoring case and punctuationCodingDecide whether a string reads the same forwards and backwards when you ignore letter case and every character that is not a letter or a digit.
Last asked Aug 2026
+2
Word breakCodingReturn whether a string can be split into a sequence of words from a dictionary, where words may be reused. Give the dynamic programming solution and its cost.
Last asked Aug 2026
Smallest number from a set of digits, with a lower boundCodingPart 1. You are given a non-empty list digits of single-digit integers (each 0-9). Use every element exactly once and arrange them to form the smallest possible non-negative…
Last asked Aug 2026
Count binary substringsCodingCount the non-empty substrings of a binary string that have the same number of 0s and 1s, with all the 0s together and all the 1s together. Solve it in one pass by comparing…
Last asked Aug 2026
Rearrange a string so no neighbours matchCodingGiven a string s of lowercase English letters, rearrange its characters so that no two adjacent characters are equal. Every character of s must be used exactly once. Implement: 1…
Last asked Aug 2026
+2
Valid parenthesesCodingGiven a string of brackets (), and {}, decide whether every bracket is closed by the matching type in the right order.
Last asked Aug 2026
Find the words in a list that are anagrams of a targetCodingGiven a target word and a list of words, return the entries that are anagrams of the target. Compare sorting each word with counting letters, and discuss case and non-letter…
Last asked Aug 2026
Hybrid run-length and bit-packing codecCodingDesign an encoder and a decoder for a list of non-negative integers. The encoder compresses the list into a list of human-readable run strings. The decoder restores the original…
Last asked Aug 2026
Valid palindrome after deleting at most k charactersCodingReturn whether a string can become a palindrome by deleting at most k characters. Relate it to the longest palindromic subsequence and give the time and space of your solution.
Last asked Aug 2026
Format bytes as a hex and ASCII dumpCodingWrite a function that prints a byte buffer as a hex dump, like xxd or hexdump -C: each line shows the offset, a fixed number of bytes in hex, and the same bytes as ASCII with…
Last asked Aug 2026
Validate and parse a bracketed list stringCodingYou are given a string s made of lowercase letters, commas, and the parentheses ( and ). A run of letters is a word. Commas separate items. A pair of parentheses wraps a group of…
Last asked Aug 2026
One-pass S-expression parser with error reportingCodingWrite a parser for a small S-expression language that reads the input character by character in a single left-to-right pass; splitting the whole string up front is not allowed. It…
Last asked Aug 2026
+3
Longest substring without repeating charactersCodingReturn the length of the longest substring with no repeated character, with a sliding window that remembers the last index of each character.
Last asked Jul 2026
Longest run of identical consecutive itemsCodingGiven a sequence, such as a string or the list of shows a user watched in order, return the longest run of the same item repeated back to back: its length and the item (and its…
Last asked Jul 2026
Count pairs of words with no letters in commonCodingGiven a list of strings, count the pairs that share no character. Encode each string's character set as a bitmask so a pair check is one AND, then group strings by mask to avoid…
Last asked Jul 2026
Count distinct strings with your own hash table, then on a GPUCodingGiven an array of strings, return how many distinct strings it contains, without using library hash sets or maps: build your own hash table with a hash function and collision…
Last asked Jul 2026
Count substrings whose characters are all distinctCodingReturn how many non-empty contiguous substrings of a string contain no repeated character, in linear time.
Last asked Jul 2026
Edit distanceCodingReturn the fewest single-character insertions, deletions and replacements that turn one string into another.
Last asked Jul 2026
Regular expression matchingCodingDecide whether a whole string matches a pattern where . matches any single character and matches zero or more of the element before it.
Last asked Jul 2026
Trie with insert, search and prefix lookupCodingBuild a trie over a dictionary of words. Support inserting a word, checking whether an exact word is present, and returning every word that starts with a typed prefix, as an…
Last asked Jul 2026
Roman numeral to integerCodingConvert a Roman numeral string to an integer. Add each symbol's value, except subtract it when a smaller value comes right before a larger one (as in IV or CM). Discuss how you…
Last asked Jun 2026
Minimal unique word abbreviationsCodingGiven a list of distinct words, give every word the shortest abbreviation that no other word shares. An abbreviation keeps a prefix of the word, then the count of skipped letters…
Last asked Jun 2026
+1
Next greater palindromeCodingYou are given a string n holding a positive integer in decimal, with no leading zeros. It can be hundreds of digits long, so it may not fit in any built-in integer type. Return…
Last asked Jun 2026
Decode waysCodingLetters are encoded as numbers, A = 1 through Z = 26. Given a string of digits, count how many ways it can be decoded back into letters. Handle zeros carefully and solve it with…
Last asked Jun 2026
+2
Group anagramsCodingGroup the words in a list that are anagrams of one another, keeping repeated words. Build the grouping key from letter counts instead of sorting each word.
Last asked Jun 2026
Find dictionary words that stretch into a given stringCodingA word can produce a string by repeating some of its letters in place: help can produce heellp. Given the string and a dictionary, return every dictionary word that could produce…
Last asked Jun 2026
Sort clothing sizesCodingGiven a list of clothing sizes such as S, M, L, XS and XL, sort them from smallest to largest. Clarify which sizes can appear (for example, how many Xs may come before S or L) and…
Last asked Jun 2026
+1
Caesar cipher with wraparound shiftsCodingImplement caesarencrypt(text: str, k: int) - str, which applies a Caesar cipher to text. Every ASCII letter moves k places forward in the alphabet. The alphabet is circular, so a…
Last asked Jun 2026
Alien dictionary: check sorted order, then infer the alphabetCoding1. You are given the order of letters in an unfamiliar alphabet and a list of words. Return whether the words are sorted according to that alphabet. 2. Now the alphabet is…
Last asked Jun 2026
Valid palindrome after deleting at most one characterCodingReturn whether a string can become a palindrome by deleting at most one character. Walk two pointers inward and, at the first mismatch, try skipping either side.
Last asked Jun 2026
Shortest unique prefix for each wordCodingGiven a list of distinct words, return for each word its shortest unique prefix: the shortest leading substring of the word that no other word in the list starts with. Return the…
Last asked Jun 2026
Find a pattern across the leaf text of a binary treeCodingEach leaf of a binary tree holds a piece of text. Read from left to right, the leaves spell one long string. Decide whether a given pattern appears in that string, including when…
Last asked Jun 2026
Implement a JSON parserCodingWrite a parser that turns a JSON string into native values: objects, arrays, strings with escapes, numbers, true, false and null. Use a recursive-descent design, skip whitespace…
Last asked Jun 2026
+1
Evaluate nested add and sub expressionsCodingImplement evaluate(expression: str) - int, which evaluates a string written in a small function-call language. An expression is either: - an integer literal, possibly negative…
Last asked Jun 2026
+1
Fill placeholders in a template stringCodingWrite a function that fills in a template string. template may contain placeholders written as {name}. The text between the braces is a variable name. keysi is a variable name and…
Last asked May 2026
+2
Minimum window substringCodingGiven strings s and t, return the shortest substring of s that contains every character of t, counting repeats, or an empty string if there is none. Grow the window until it…
Last asked May 2026
Longest common prefixCodingGiven a list of strings, return the longest prefix they all share (empty if none). Compare character by character across the strings, or compare only the smallest and largest…
Last asked May 2026
Build a target string from the fewest substrings of a sourceCodingExpress a target string as a concatenation of substrings of a source string, each given by its index range, using as few pieces as possible. Follow-up: after one character of the…
Last asked May 2026
Count palindromic substringsCodingCount the substrings of a string that are palindromes, counting each position separately. Expanding around every centre gives O(n^2) time and O(1) space.
Last asked May 2026
Per-position feedback for a guess against a secret wordCodingGiven a secret word and a guess of the same length, return feedback for each position of the guess, Wordle style: the letter is in the right place, is in the word but elsewhere…
Last asked May 2026
Word search in a straight lineCodingGiven an m x n grid of uppercase letters board and a string word, decide whether word appears in the grid along a straight line. A match starts at some cell and reads len(word)…
Last asked May 2026
Store name autocompleteCodingImplement autocomplete over a set of store names: given the prefix typed so far, return the matching stores. Discuss a trie against simpler options, how many results to return and…
Last asked May 2026
Parse and summarize HTTP access logsCodingYou get log lines, each meant to hold an IP address, a timestamp, a URL or API path and a status code. Parse each line, decide whether it is valid, and report the total number of…
Last asked May 2026
Remove invalid parentheses: every minimal resultCodingRemove the minimum number of parentheses from a string so it becomes valid, and return every distinct valid string you can get that way.
Last asked Apr 2026
Deduplicate account sign-ups by email and phoneCodingA sign-up service processes account registrations one at a time, in the order they arrive. You get three lists of the same length n: names, emails and phones. Index i of each list…
Last asked Apr 2026
Quickselect, distance between tree nodes, and substrings with a prefixCodingThree problems from one loop: 1. Find the k-th largest element with quickselect. Compare the approaches and how to choose the pivot, then implement one. 2. Given the edges of a…
Last asked Apr 2026
Count words in a text streamCodingWrite a class that receives text in chunks from a stream and keeps a count of how many times each word has appeared, with a method to read the count of a word (or the most common…
Last asked Apr 2026
Live string editing with a mouse cursorCodingImplement live string editing operations positioned by a mouse cursor. Reported on 1point3acres' Harvey bank; reconstructed in our own words.
Last asked Apr 2026
Count ways to spell a word with element symbolsCodingGiven the symbols of the chemical elements and a word, count the ways to write the word as a sequence of element symbols, ignoring case. For example, "Physics" splits as…
Last asked Apr 2026
Find words that contain another word from the listCodingGiven a list of strings, find the strings that contain another string from the same list as a substring. For "programming", "am", "pro" the answer is programming. Start from the…
Last asked Apr 2026
Snake case to camel case, then across a whole repositoryCoding1. Convert a valid snakecase name (no leading, trailing or doubled underscores) to lowerCamelCase, so foobar becomes fooBar. 2. An open discussion: a huge repository names its…
Last asked Apr 2026
Longest ON run with k range flipsCodingA data center rack holds n servers in a row. Their states are given as a binary string serverstates, where '1' means the server is ON and '0' means it is OFF. An operator may run…
Last asked Apr 2026
One edit distanceCodingReturn whether two strings are exactly one edit apart, where an edit inserts, deletes or replaces one character. Solve it in one pass and clarify the edge cases (equal strings…
Last asked Apr 2026
Shift security units left to protect the most populationCodingCities stand in a row, numbered 0 to n - 1. You are given an integer array population of length n (all values = 0) and a binary string unit of the same length. uniti == '1' means…
Last asked Apr 2026
Valid numberCodingReturn whether a string is a valid number: an integer or decimal with an optional sign, optionally followed by an exponent (e or E and an integer). Spell out the rules first, then…
Last asked Apr 2026
Split Markdown into chunks that keep their headersCodingSplit a Markdown file into chunks no larger than a size limit. When a new chunk starts in the middle of a section, it must begin with the headers that text sits under (for example…
Last asked Apr 2026
Can one board be turned into another by moving pieces?CodingA board is a string of R, L and empty cells. R pieces can only move right and L pieces only left, one empty cell at a time, and no piece can jump over another. Decide whether a…
Last asked Apr 2026
Substring search and weighted random samplingCodingTwo problems for a machine learning engineer screen: 1. Find where a pattern occurs in a text, and discuss doing better than checking every position. 2. Pick an item at random…
Last asked Mar 2026
Add bold tags around matching substringsCodingGiven a string and a list of words, wrap every part of the string that matches any word in bold tags. Overlapping or touching matches share one pair of tags. Mark the covered…
Last asked Mar 2026
Score how close a string is to a palindromeCodingDesign a score for strings that are almost palindromes and implement it. Decide what the score measures (for example, mismatched pairs when comparing from both ends, or the fewest…
Last asked Mar 2026
Is every 3+ letter piece of a string a dictionary word?Coding1. Given a string and a dictionary, return true when every piece of the string of length 3 or more is a valid word. (The report says subsequences; confirm with the interviewer…
Last asked Mar 2026
+2
Truncate model output at the first stop sequenceCodingImplement a function that returns the part of a string that comes before the earliest stop word. - text is any string. It may be empty. - stopwords is a list of non-empty strings…
Last asked Mar 2026
Letter combinations of a phone numberCodingEach digit from 2 to 9 on a phone keypad stands for a few letters. Given a string of digits such as "2345", list every letter string it can spell and say how many there are.
Last asked Mar 2026
Unique Morse code wordsCodingTranslate each word into Morse code by joining the codes of its letters, and return how many different translations there are.
Last asked Feb 2026
+1
Grep with context linesCodingImplement a simplified grep. You get a list of text lines, a search string target, and a non-negative integer k. Return every line that contains target, together with up to k…
Last asked Feb 2026
Minimum remove to make valid parenthesesCodingA string holds letters and parentheses. Remove the fewest parentheses so that the rest is balanced, and return any valid result. Do it with a stack of indices first, then without…
Last asked Feb 2026
Most coins collected by tokens that jump three cellsCodingA one-player board game: a string of N positions, each empty (.), holding one of the player's tokens (T) or holding a coin (C). The player may have several tokens, and a token…
Last asked Feb 2026
Render, compress and invert a bitmap fontCodingA bitmap font is given as strings of 0 and 1 bits, one bitmap per character. 1. Render each character as a grid of text, with . for a 0 bit and another character for a 1 bit. 2…
Last asked Feb 2026
Parse logs and count error codesCodingRead lines of a log, pull out the error code from each line that has one, and count how often each code appears. Skip malformed lines.
Last asked Feb 2026
Shortest run-length encoding after deleting k charactersCodingRun-length encoding writes each run of a repeated character as the character followed by the run length when the run is longer than one, so "aaabcc" becomes "a3bc2". Given a…
Last asked Feb 2026
Fewest lines to wrap a sentenceCodingGiven a non-empty string text and a positive integer maxwidth, return the fewest lines needed to lay out all the words of text. Implement: Rules: - A word is a run of non-space…
Last asked Feb 2026
Decode a string with nested repeat countsCodingAn encoded string uses k... to mean the bracketed part repeated k times, and brackets may nest, for example 3a2c decodes to accaccacc. Return the decoded string. Use a stack or…
Last asked Feb 2026
Smallest number from an increase and decrease patternCodingGiven a pattern of I (increase) and D (decrease) characters of length n, build the smallest number of n + 1 distinct digits from 1 to 9 whose consecutive digits follow the…
Last asked Feb 2026
Remove dictionary words from a character stream as they appearCodingCharacters arrive one at a time. Whenever the characters kept so far end with a word from a given dictionary, remove that word, and keep going (a removal can make a new match…
Last asked Feb 2026
Shortest unique abbreviation for every wordCodingAbbreviate each word in a list of distinct words as a prefix, the count of the letters in between, and the last letter (like becomes l2e). When two words share an abbreviation…
Last asked Feb 2026
String compression and decompressionCodingWrite a function that compresses a string (for example with run-length encoding, aaabcc to a3b1c2 or a variant the interviewer gives) and one that decompresses it back exactly…
Last asked Jan 2026
Weighted 01 and 10 subsequence pairs with wildcardsCodingA status string holds '0', '1' and '!'. Every pair of positions i < j that reads 0 then 1 (a subsequence, not a substring) costs x, and every pair that reads 1 then 0 costs y…
Last asked Jan 2026
+1
Bracket the first matching substring in each wordCodingImplement wrapsubstrings(s: str, elements: liststr) - str. s is a sentence of words separated by single spaces. elements is a list of strings. For each word in s, find the first…
Last asked Jan 2026
Validate an IPv4 addressCodingGiven a string, decide whether it is a valid IPv4 address: four decimal numbers from 0 to 255 separated by dots, with no leading zeros and nothing else in the string.
Last asked Jan 2026
Minimum add to make parentheses validCodingReturn the fewest parentheses to insert so a string of ( and ) becomes balanced, counting unmatched openers and closers in one pass.
Last asked Jan 2026
Best-matching binary pattern with wildcardsCodingYou get a list of patterns over 0, 1 and a wildcard that matches either bit, and a query string of bits. Return the pattern that matches the query best. Agree on what "best" means…
Last asked Jan 2026
Word break II: every way to split a sentenceCodingGiven a string and a dictionary of words, return every sentence you can form by splitting the string into dictionary words, with words separated by spaces. Words may be reused…
Last asked Jan 2026
Link similar user records by weighted field matchesCodingA data-quality team wants to find user profiles that probably belong to the same person. You get records, a 2D list of strings. The first row is a header of field names, and one…
Last asked Jan 2026
Add two binary stringsCodingGiven two binary numbers as strings, return their sum as a binary string, adding digit by digit from the right with a carry, without converting to integers.
Last asked Jan 2026
Repeatedly remove runs of equal adjacent charactersCodingRemove every run of two or more equal adjacent characters, and keep doing it as new runs form, until none remain. For example xyyyx becomes xx and then the empty string. Do it in…
Last asked Jan 2026
Longest palindromic subsequenceCodingReturn the length of the longest subsequence of a string that reads the same forwards and backwards. Give the interval dynamic programming and its O(n^2) time.
Last asked Dec 2025
Four short array and string exercisesCodingFour quick problems in one round: 1. Return the difference between the number of uppercase and lowercase letters in a string. 2. Repeatedly subtract from the non-zero elements of…
Last asked Dec 2025
Insert plus or minus signs between digits to reach a targetCodingGiven a string of digits and a target, insert +, - or nothing between consecutive digits so the expression equals the target, and return every such expression. Handle numbers with…
Last asked Dec 2025
Generator for substring pattern matchesCodingWrite a generator that takes a string and a pattern and yields each place where a substring of the string matches the pattern, one at a time as it scans. Discuss the cost of the…
Last asked Dec 2025
Word ladderCodingGiven a start word, an end word and a word list, find the length of the shortest sequence from start to end in which each step changes exactly one letter and every word after the…
Last asked Dec 2025
Longest duplicated substringCodingReturn any longest substring that occurs at least twice in a string (the occurrences may overlap), or an empty string if none does. Binary search on the length with a rolling…
Last asked Dec 2025
Collect coins with tokens that move three steps rightCodingA one-row board is a string of . (empty), T (a token) and C (a coin). A move takes any token exactly three cells to the right, onto a cell no token occupies; landing on a coin…
Last asked Dec 2025
Longest palindromic substringCodingReturn the longest substring of a string that reads the same forwards and backwards. Start with expanding around each center, state the time and space, and say how you would get…
Last asked Nov 2025
Shortest substring with N distinct charactersCodingGiven a string and a number n, return the shortest substring that contains n distinct letters (or say that none exists). Use a sliding window that grows until it holds n distinct…
Last asked Oct 2025
Fewest guesses in Wordle with green feedback onlyCodingA secret five-letter word must be found, and the only feedback is, for a guess, which positions hold the right letter. Design a guessing strategy that finds the word in as few…
Last asked Oct 2025
Valid palindrome IICodingReturn whether a string can become a palindrome after deleting at most one character. Walk two pointers inward; at the first mismatch try skipping either side once.
Last asked Oct 2025
Group shifted stringsCodingTwo strings belong together if shifting every letter of one by the same amount (wrapping z to a) gives the other. Group a list of strings by this relation, keying each string by…
Last asked Sep 2025
Remove runs of repeated characters (1D candy crush)CodingRepeatedly remove every run of at least k equal adjacent characters from a string until none is left, and return what remains. Removing a run can join two runs into a new one…
Last asked Sep 2025
Card number validation with the Luhn checkCodingImplement a payment-card validator. Check a card number with the Luhn checksum and work out its network from its prefix and length. Later parts recover card numbers with missing…
Last asked Sep 2025
Earliest time a password becomes irrecoverableCodingA password is attacked one character at a time: you get the order in which positions are replaced by '', one per second. The password counts as irrecoverable once at least m of…
Last asked Sep 2025
Serialize and deserialize a trieCodingDesign and implement serialize and deserialize for a dictionary trie, so that the trie can be written to a string and rebuilt exactly, including which nodes mark the end of a…
Last asked Sep 2025
Remove deleted folders and their descendants from a path listCodingYou get a list of hierarchical paths such as California, California/San Francisco and California/San Francisco/7th Street, and a list of paths to delete. Deleting a path removes…
Last asked Sep 2025
+1
Count the words in a documentCodingGiven the text of a document, return how many words it contains. The real work is the definition: agree with the interviewer what counts as a word (is "it's" one word or two, what…
Last asked Sep 2025
Autocomplete with a trieCodingBuild an autocomplete: insert words (optionally with counts) into a trie, and for a prefix return the matching words, for example the most popular few. Make sure results are…
Last asked Sep 2025
Sort strings by length, then merge two sorted listsCodingGiven two unsorted lists of strings, sort them by string length. Follow-up: if both lists already arrive sorted by length, merge them into one sorted list in linear time.
Last asked Sep 2025
Equalize two strings' letter counts by deleting one letterCodingIn each of two strings you pick one letter and delete any number of its occurrences (possibly none). Decide whether the two strings can end up similar, meaning every letter…
Last asked Sep 2025
Autocomplete with top-k suggestionsCodingGiven a list of past queries with their counts, build an autocomplete: for a prefix the user has typed, return the k most popular completions. Use a trie, and discuss storing top…
Last asked Sep 2025
Validate a bracket string after digit-driven deletionsCodingA string mixes brackets and digits, for example (())2)1(). Each digit d requires removing exactly d of the brackets that come before it. Decide whether the deletions can be chosen…
Last asked Aug 2025
Valid palindrome IIICodingReturn whether a string can become a palindrome by deleting at most k characters. Relate it to the longest palindromic subsequence and solve it with dynamic programming.
Last asked Aug 2025
Palindrome pairsCodingGiven a list of distinct words, return every pair of indices (i, j) where wordsi + wordsj is a palindrome. Checking all pairs is too slow; put the reversed words in a hash map (or…
Last asked Aug 2025
Split item categories into a prefix and suffix that share enoughCodingSale items form a string itemCategories, one letter per item's category. Count the ways to cut it into a non-empty prefix and a non-empty suffix such that more than k distinct…
Last asked Aug 2025
Substring with concatenation of all wordsCodingGiven a string and a list of words of equal length, return every start index where a substring is some concatenation of all the words, each used exactly once. Use a sliding window…
Last asked Jul 2025
Add two decimal number stringsCodingAdd two non-negative numbers given as strings and return the sum as a string, digit by digit with a carry. Then support a decimal point: pad the fractional parts to the same…
Last asked Jul 2025
Palindrome permutationCodingReturn whether some rearrangement of a string is a palindrome: at most one character may occur an odd number of times. Follow-up: build and return one such palindrome.
Last asked Jul 2025
How strings are stored and what copying them costsCodingWhat is a string under the hood? Describe the fields a string struct holds and how you would implement one yourself, then give the time cost of copying a string and of common…
Last asked Jul 2025
Multiply stringsCodingMultiply two non-negative integers given as strings and return the product as a string, without converting them to built-in big integers. Use grade-school multiplication into an…
Last asked Jul 2025
Compare two linked lists of string chunksCodingTwo singly linked lists each hold pieces of text, one piece per node (in the reported version, one character per node stored as a string). Decide whether both lists spell the same…
Last asked Jul 2025
Fewest flips to split a binary string into even uniform blocksCodingA string of 0s and 1s must be cut into blocks of even length where each block is all 0s or all 1s. Find the fewest bit flips that make this possible, and among those, the fewest…
Last asked Jul 2025
Valid palindrome and its variantsCodingDecide whether a string is a palindrome after ignoring case and anything that is not a letter or digit. Then solve variants: allow deleting at most one character, or check without…
Last asked Jul 2025
Can left and right pieces slide into a target stringCodingA string holds pieces that can only move left, pieces that can only move right, blanks, and walls that nothing passes. Given a start and a target string, decide whether the start…
Last asked Jul 2025
First unique character in a stringCodingReturn the index of the first character in a string that appears only once, or -1 if every character repeats. Do it in two passes with a count table.
Last asked Jul 2025
Predictive text for a phone keypadCodingOn a phone keypad each digit maps to several letters (2 = abc, 3 = def, ...). Given a dictionary and a sequence of digit presses, return the words the sequence could spell, T9…
Last asked Jun 2025
Shortest distance between X and Y in a stringCodingFind the shortest distance between characters X and Y in a string in one pass. Reported on 1point3acres' Glean bank; reconstructed in our own words.
Fill a weekly work schedule to an exact hour totalCodingA week's work plan is written as a 7-character string pattern, one character per day. A digit (0-9) is a day whose hours are already set. A ? is a day that is still open…
Match words against a letter patternCodingGiven a pattern string and a list of words, decide for each word whether it follows the pattern. A word follows the pattern when it has the same length as the pattern and the…
Count substrings that use only vowels and contain all fiveCodingGiven a string s of lowercase English letters, return the number of contiguous substrings that satisfy both rules: 1. Every character of the substring is a vowel (a, e, i, o, u)…
Template formatter with plural blocksCodingImplement formattemplate(text: str, keys: liststr, values: liststr) - str. keysi maps to valuesi; all values are strings. text can contain two kinds of expressions in braces: 1…
Co-occurring tags for a tag filterCodingEach title in a catalog carries a group of tags that were assigned together, for example "drama", "british", "crime". You get all the groups as a list of lists of strings, groups…
Smallest palindrome from a symmetric string's lettersCodingYou are given a string s of lowercase English letters that is already a palindrome. Rearrange all of its characters to form a new palindrome, and return the one that comes first…
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Remove parentheses from a plus/minus expressionCodingWrite a function simplify(expr: str) - str that removes every parenthesis from an expression and gives each term the sign it really has. Input. A valid expression made of: -…
Regex matcher with starred groupsCodingImplement a small regex engine for a text-processing pipeline. Write ismatch(s: str, pattern: str) - bool. It returns True only when the whole string s matches pattern. The…
Count recipes a pantry can coverCodingYou keep a pantry of ingredients, written as a string pantry. Each character is one unit of an ingredient, so "aab" holds two units of a and one unit of b. You also have a list of…
Split a string into blocks that match the prefix's letter countsCodingYou get a string s of uppercase letters A-Z. Call a positive length L valid when: - L <= len(s) and s can be cut into consecutive, non-overlapping blocks of exactly L characters…
Minimum adjacent swaps to group red ballsCodingA row of balls is given as a string balls. Each character is 'R' (red) or 'W' (white). In one move you may swap two neighbouring balls. Return the minimum number of moves needed…
Largest number by changing up to k digits to 5CodingYou are given a string s that holds a positive integer (digits only, no leading zero) and an integer k. In one operation you pick any digit of s that is not 5 and change it to 5…
Count letters whose lowercase copies all come before uppercaseCodingGiven a string s of English letters (it may also contain other characters, which you ignore), count the distinct letters, compared case-insensitively, that satisfy this rule:…
Longest valid bracket substring with nesting ranksCodingGiven a string made only of the characters (, ), , , { and }, return the length of the longest contiguous substring that is valid. A string is valid when all of these hold: 1…
Longest substring without three repeats in a rowCodingGiven a string s, return the length of the longest substring of s in which no character appears three or more times in a row. Call such a substring semi-alternating: runs of one…
Longest proper prefix that is also a suffixCodingGiven a string s, return the length of the longest string that is both a proper prefix and a proper suffix of s. - A prefix is a run of characters at the start of s; a suffix is a…
Longest subsequence of one string that is a substring of anotherCodingGiven two strings x and y, return the length of the longest string t such that: - t is a subsequence of x: its characters appear in x in the same order, but not necessarily next…
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Multi-tap keypad: every string a press sequence can spellCodingAn old phone types letters by multi-tap: pressing a key once gives its first letter, twice in a row gives its second letter, and so on. Use this keypad: Key Letters ------ 1 A B C…
Integer to English words, built up by rangeCodingWrite a function that spells out an integer in English words. The interviewer adds the ranges in parts, and later parts should reuse earlier ones, so design for extension from the…
Justify articles to a fixed line widthCodingYour platform shows articles in a fixed-width text column. Write a function that lays out a list of articles and returns every output line in order, as one flat list. Rules - Each…
Spell a word on a 5-column letter boardCodingA text-entry device shows the 26 lowercase letters on a board 5 columns wide, filled row by row. Letter c sits at row (ord(c) - ord('a')) // 5 and column (ord(c) - ord('a')) % 5:…
Count reverse-complement DNA strand pairsCodingYou are given a list of DNA strands. Each strand is a non-empty string over the alphabet A, C, G, T. The reverse complement of a strand is formed by reversing it and then…
Count redundant substrings by vowel and consonant weightsCodingWrite a function that counts the redundant substrings of a word. A string s is redundant when its length equals a V + b C, where V is the number of vowels in s, C is the number of…
Fewest append-to-end moves to reverse a binary imageCodingAn image is stored as a binary string s: '1' is a white pixel and '0' is a black pixel. To make a backup, you want to turn s into its reverse, s read from right to left. You may…
Sort words by a custom alphabetCodingYou are given a string order that lists characters from first to last in a custom alphabet. Each character appears in order at most once. You are also given a list of strings…
Count distinct passwords from one substring reversalCodingA security team scores a password by its variability. Variability is the number of distinct strings you can get by reversing exactly one contiguous substring of the password…
Password similarity under cyclic character bumpsCodingA security team wants to reject a new password when it is too close to the user's previous one. You may transform the new password like this: pick any set of its positions…
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Longest palindrome from equal-length wordsCodingYou are given a list of words. Every word has the same length k and uses lowercase letters. The list can contain duplicates. Pick some of the words (possibly none) and join them…
Merge known bigrams in a sentence, leftmost firstCodingWrite a function that splits a sentence into tokens using a dictionary of known two-word phrases (bigrams). - text is a non-empty string of lowercase words separated by spaces. -…