Key-value cache with per-entry TTLCodingImplement an in-memory key-value cache where every write gives a key, a value and a time-to-live. Once an entry's time is up it must behave as if it was never written. Support…
Last asked Oct 2026
#hash maps
Maximum frequency stackCodingBuild a stack of integers with push(x) and pop(). pop removes and returns the value that occurs most often in the stack; when several values tie, it takes the one with an…Last asked Oct 2026
Insert, delete and get random in O(1) with duplicatesCodingDesign a collection of integers that allows duplicates and supports, each in average O(1) time: 1. insert(val): add one copy of val; return whether val was absent before. 2…Last asked Sep 2026- Build reporting trees from employee-manager pairsCodingYou get a list of employee, manager pairs, where each person is a positive integer id and the pair means the employee reports directly to that manager. Build the org chart…Last asked Sep 2026
Find timed-out RPC calls from start and end logsCodingA service writes one log record when a remote procedure call starts and another when it ends, and the log is in time order. Given the log and a timeout, report every call that ran…Last asked Sep 2026- Largest rectangle from a point set, any orientationCodingYou get distinct points with integer coordinates. Choose four of them that are the corners of a rectangle, where the sides do not have to be parallel to the axes. Return the…Last asked Sep 2026
Streaming top-K frequent elements with O(1) updatesCodingElements arrive one at a time from an unbounded stream. Design a structure that can report, at any moment, the K elements seen most often so far. Clarify what K is and whether it…Last asked Sep 2026
Binary tree vertical order traversalCodingReturn a binary tree's values column by column, from the leftmost column to the rightmost. The root is in column 0, a left child one column to the left of its parent and a right…Last asked Sep 2026- Two sumCodingGiven an integer array and a target, return the indices of the two different elements that add up to the target; exactly one such pair exists. Then solve it without a hash map…Last asked Sep 2026
- +1Sparse matrix multiplicationCodingMultiply two matrices that are mostly zeros. Store only the non-zero entries and skip work on zeros, and state the cost in terms of the number of non-zero entries.Last asked Sep 2026
Link object tracks across two perception systemsCodingTwo perception systems, A and B, track the same objects around a vehicle independently, each with its own IDs, and each emits observations of its tracks over time. Implement an…Last asked Sep 2026- Most active user by distinct contacts, then top KCodingUsers send each other messages, and a message between A and B counts as contact for both of them. Build a class with: - register(a, b): record a message between a and b. -…Last asked Sep 2026
Per-key fixed-window cost limiterCodingBuild a rate limiter that meters cost rather than request count. Each key may spend at most limit units in each fixed window of window seconds, with windows aligned to multiples…Last asked Sep 2026- Is a vehicle ready to go after its maintenance events?CodingA vehicle can leave only when all its maintenance work is closed. Its log is a list of events: each opens a maintenance task of some type or ends the open task of that type…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
- Duplicate detection: sliding windows, then read-heavy queriesCodingStart with checking an array for duplicate values, then evolve the solution step by step: first find duplicates that fall within a sliding window of positions, then rework the…Last asked Sep 2026
- +5Top K frequent elementsCodingGiven an integer array and k, return the k values that occur most often, in any order; ties at the cutoff may go either way. One version asks for the most frequent request IDs in…Last asked Sep 2026
- +2Copy list with random pointerCodingEach node of a singly linked list has a next pointer and a random pointer that may point to any node in the list or to nothing. Return a deep copy of the list in which no pointer…Last asked Aug 2026
- Earliest pair with a target sumCodingGiven an array of numbers and a target, find the pair of elements that sums to the target and appears earliest in the array.Last asked Aug 2026
- Count customers who visited on more than one dateCodingGiven visit records of a customer and a date, count the customers who visited on at least two different dates.Last asked Aug 2026
- Subarray sum equals kCodingCount the contiguous subarrays whose sum equals k; the array may contain negative numbers. Follow-ups: only report whether such a subarray exists, or return the longest one.Last asked Aug 2026
- Unique and most frequent keys in a recent time windowCodingEvents arrive with a key and a timestamp. Build a structure that, for the most recent window of time, answers which keys appeared exactly once and which key appeared most often…Last asked Aug 2026
- +3Longest 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
- 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
- Find every set in a hand of Set cardsCodingEach card in the game Set has four attributes (color, shape, number, shading), and each attribute takes one of three values, so a card is four numbers from 0 to 2. Three cards…Last asked Jul 2026
- Thread-safe tally counter with bounded memoryCodingBuild a tally service that many threads call at once to count occurrences of keys, and that answers queries for counts. Memory is bounded, so it cannot keep every key forever…Last asked Jul 2026
- +2Group 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
- Most common website visit patternCodingGiven which user visited which website and when, find the sequence of three websites that the most users visited in that order.Last asked May 2026
- +2Minimum 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
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- Merge two named treesCodingEach tree node is a dictionary with a name, an integer value and an ordered list of children. Given two such trees t1 and t2, build a new merged tree that follows t1's nodes and…Last asked May 2026
- Searchable sentence indexCodingBuild an index over a collection of sentences so you can quickly find the sentences that contain given words.Last asked May 2026
Last-click attributionCodingImplement APIs that record users' ad clicks and, when a user converts, credit the conversion to that user's most recent click. Settle the open questions first, such as an…Last asked May 2026- +2Insert, delete and get random in O(1)CodingDesign a set with insert(val), remove(val) and getrandom(), each in average O(1) time. getrandom returns each current element with equal probability.Last asked May 2026
- Unique pairs with a target sumCodingReturn each distinct pair of values from an array that sums to a target, counting a pair only once even when the values repeat.Last asked May 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
- Process login logs split across shardsCodingLogin records are spread across several shards. Combine them to answer questions about users' logins, merging the shards correctly and efficiently, and explain how your approach…Last asked Apr 2026
- Maximum sum of a subarray whose ends differ by kCodingGiven an integer array and k, a subarray is good when its first and last elements differ by exactly k. Return the largest sum of a good subarray, or 0 if there is none.Last asked Apr 2026
- Build a binary tree from parent-child descriptionsCodingEach description gives a parent value, a child value, and whether the child is the left or right child. Build the binary tree and return its root.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
- Message latency from send and receive logsCodingTwo CSV files describe messages between two machines: one lists when machine A sent each message, the other when machine B received it. Match the rows by message and compute each…Last asked Mar 2026
- Restaurant waitlist managerCodingBuild a waitlist for a restaurant: parties join with their size, may leave before being seated, and when a table frees up you seat the party that should get it. Keep each…Last asked Mar 2026
- Manager and peer relationships with is_manager queriesCodingImplement three functions: - manager(a, b): a is the manager of b. - peer(a, b): a and b have the same manager. - ismanager(a, b): return whether a is a manager of b. Decide how…Last asked Feb 2026
- Worker hours registerCodingImplement a register that records the hours workers log and answers questions about each worker's totals.Last asked Feb 2026
- Next-word predictor from bigram countsCodingYou get training sentences as lists of words, for example "a", "b", "c", "a", "s", "d". 1. Count, for every word, which words follow it and how often. 2. predict(word) returns the…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- Top K frequent elements and K closest pointsCodingTwo warm-up style problems that the interviewer combined into one round: 1. Return the k points closest to the origin. 2. Return the k most frequent values in a list. Then combine…Last asked Feb 2026
- Dot product of two sparse vectorsCodingStore very large, mostly-zero vectors compactly and compute the dot product of two of them. Compare a hash map with sorted index-value pairs, and say how you would reuse work when…Last asked Feb 2026
- Top-k and bottom-k words in a list and in a streamCoding1. Given a list of words, return the k most frequent and the k least frequent. 2. Now the words arrive as a stream: keep both answers available at any moment. 3. Discuss how to…Last asked Jan 2026
- Logger that reports its top K messagesCodingBuild a logger that receives messages over time and can return the k most frequent messages at any point. Choose structures so logging stays fast and the top-K query is efficient…Last asked Jan 2026
- Recover the original array from its doubled shuffleCodingAn array was built by taking an original array, appending twice each of its values, and shuffling the result. Given the shuffled array, return the original, or an empty result if…Last asked Jan 2026
- Set with constant-time insert, delete and get randomCodingDesign a set with add, remove and getrandom, all in average O(1); getrandom returns each element with equal probability. Handle empty-set and missing-element cases. Follow-up: how…Last asked Jan 2026
- Two sum with the lexicographically smallest index pairCodingGiven an integer array and a target, return a pair of indices i < j whose values add up to the target. When several pairs work, return the lexicographically smallest one (smallest…Last asked Dec 2025
- Top three categories by total scoreCodingYou get a list of (category, score) tuples. Add up the score of each category and return the best total you can get from at most three categories (the three highest totals)…Last asked Dec 2025
- Top K most active users from chat logsCodingYou get chat log lines, each naming the users involved. Return the k users who took part in the most chats. Count with a hash map, then compare picking the top k by sorting, by a…Last asked Dec 2025
- Thread-safe job registry with expiryCodingBuild an in-memory registry of jobs that many threads read and write at once. Each job is registered with a time to live; lookups must not return a job whose time has passed, and…Last asked Nov 2025
- Settle group trip expenses with the fewest transfersCodingFriends on a trip record payments: each record names the payer, the amount and the people it covered (the payer may be among them), split evenly. Compute each person's net…Last asked Nov 2025
- Contiguous array with equal 0s and 1sCodingGiven a binary array, return the length of the longest contiguous subarray with as many 0s as 1s. Treat 0 as -1 and look for two equal prefix sums as far apart as possible.Last asked Nov 2025
- Count subarrays with at least k equal pairsCodingGiven an integer array and k, count the contiguous subarrays that contain at least k pairs of indices i < j with equal values. For example, 0, 1, 0, 1, 0 with k = 2 has 3 such…Last asked Sep 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
- Counter class with add, count and removeCodingWrite a class that stores strings with how many times each was added. Support add(s), count(s) and remove(s). Follow-ups: return the total count across everything stored, and how…Last asked Sep 2025
- Validate checkout and return logsCodingGiven a log of events per book, each a checkout or a return (a true/false flag), check that every book's events alternate correctly: a book must be checked out before it can be…Last asked Sep 2025
- Count user sessions from an activity logCodingEach log line is timestamp, user, action. A session is a run of a user's actions with no gap longer than a given timeout. Return the number of sessions per user. Parse robustly…Last asked Sep 2025
- Contains duplicate within distance KCodingReturn whether an array has two equal values whose indices differ by at most k. Start with a map from value to its last index. Follow-up: cut the extra space to O(k) by dropping…Last asked Aug 2025
- LRU cache where each item counts by its quantityCodingImplement an LRU cache in which every item carries a quantity, and capacity is measured in total quantity rather than number of items. A put may evict several least recently used…Last asked Aug 2025
- Construct binary tree from inorder and postorder traversalCodingGiven the inorder and postorder traversals of a binary tree with distinct values, rebuild the tree. The last postorder value is the root; its position in the inorder list splits…Last asked Aug 2025
- Recommend accounts your followees followCodingGiven a dictionary from each user to the people they follow, for example {A: B, C, B: C, D, C: E}, return for a user every account followed by the people they follow that they…Last asked Aug 2025
- LFU cacheCodingImplement a least-frequently-used cache with get and put in O(1). When full, evict the key used least often, breaking ties by least recently used. Keep a map from key to node and…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
- Insert delete getrandom O(1)CodingDesign a set with insert, remove and getRandom (each element equally likely), all in average constant time, using an array of values and a map from value to index; remove by…Last asked Jul 2025