#counting

34 counting interview questions, asked at Amazon, Microsoft, Google and 11 more companies. Last asked Oct 2026.

Count ordered routes that visit one shop of each of four typesCodingYou need four things from four kinds of shop: a bookshop, a school-supply shop, a sports store and a stationery store. The town has many shops of each kind, joined by roads…
Last asked Oct 2026
+1
Friend recommendation by mutual friendsCodingRecommend friends ranked by number of mutual friends in a social graph. Reported on 1point3acres' Fireworks AI bank; reconstructed in our own words.
Last asked Sep 2026
Count squares formed by vertical and horizontal segmentsCodingYou get a set of vertical and a set of horizontal line segments. Every point where a vertical and a horizontal segment cross or touch is an intersection. Count the squares whose…
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
Fewest stones left after merging equal-level pairsCodingEach stone has a positive integer level. Two stones of the same level x can be merged into one stone of level x + 1, and merged stones can be merged again. Merging as much as you…
Last asked Sep 2026
+7
Design a top-K popular items systemSystem designReport the K most viewed or played items over sliding windows (last minute, hour, day) from a firehose of billions of events per day.
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
Minimum adjacent swaps to group a binary arrayCodingGiven an array of 0s and 1s, return the fewest swaps of neighbouring elements that put all the 0s at one end and all the 1s at the other. Either order is allowed.
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
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
Count non-decreasing arrays with given digit sumsCodingYou get an array requiredSum of length n. Count arrays result of length n that are non-decreasing and where the digit sum of resulti equals requiredSumi for every i, within the…
Last asked Aug 2026
Is a 14-tile mahjong hand a winning hand?CodingGiven 14 mahjong tiles, decide whether they form a winning hand: four sets, each three identical tiles or three consecutive tiles of the same suit, plus one pair.
Last asked Mar 2026
Largest subset of two-digit numbers sharing a digitCodingAn array holds only two-digit integers (10 to 99). Choose as many elements as you can such that every chosen number contains at least one common digit. Return the count; for 52…
Last asked Feb 2026
+1
Crack a 4-digit code from exact-match countsCodingA server holds a secret 4-character code. Each character is a digit from '0' to '9'. Digits may repeat, and the code may start with zeros. You can only learn about the code…
Last asked Feb 2026
H-index of instance ratingsCodingGiven the integer ratings of a set of instances, return the largest integer k such that at least k instances have a rating of k or more (return 0 when no such k exists). Do it…
Last asked Jan 2026
Count three-digit numbers with distinct digitsCodingCount the three-digit numbers whose digits are all different, first by brute force and then by counting directly.
Last asked Jan 2026
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
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
Fleet capacity after server id replacementsCodingA service runs on a fleet of n servers. Each server has an integer id, and a server with id x can handle x requests at a time. The fleet's total capacity is the sum of all server…
Last asked Sep 2025
Task scheduler with a cooldown between identical tasksCodingGiven a list of tasks (letters) and a cooldown n, identical tasks must be at least n time units apart; the CPU can idle. Return the fewest time units to run them all. Solve it…
Last asked Jul 2025
Target indices after sorting an arrayCodingReturn the indices at which a target would sit once the array is sorted. In the reported version the array is already sorted and the answer must take logarithmic time: binary…
Last asked Jul 2025
Fewest passes to collect a permutation in orderCodingA shelf holds a permutation of 1..n. Starting with target 1, you scan the shelf left to right, and whenever you see the current target you take it and look for the next number…
Last asked Jul 2025
Count index pairs with an even product and odd distanceCodingGiven an integer array, count pairs i < j where numsi numsj is even and j - i is odd. Avoid checking every pair: count even and odd values by index parity and combine.
Last asked Jan 2025
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)…
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…
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…
Rearrange house numbers into sorted, duplicate-free neighborhoodsCodingA city is a list of neighborhoods. Each neighborhood is a list of house numbers (integers), and numbers may repeat, both within a neighborhood and across the city. Move houses…
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:…
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…
Largest subset with a non-zero bitwise ANDCodingGiven an array arr of positive integers, choose as many elements as you can so that the bitwise AND of all chosen elements is greater than 0. The chosen elements can come from…
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…
Build the longest spike from an arrayCodingA spike is a sequence of integers that strictly increases up to a single peak and then strictly decreases. The peak belongs to both parts, and either part may be empty, so a…