#hashing

52 hashing interview questions, asked at Amazon, Microsoft, Google and 14 more companies. Last asked Oct 2026.

Check and insert among a million user IDsSystem designA service holds about a million user IDs. Each request carries a user ID: if it is not in the set yet, add it. Decide how to store the IDs and run this check-and-insert, and talk…
Last asked Oct 2026
+2
Design an image sharing service that stores each file onceSystem designDesign a service where users upload, view, download and delete images, and identical images are stored only once, keyed by a content hash. Explain how metadata and blobs stay…
Last asked Sep 2026
+3
Deduplicate files by size then content hashCodingWalk a directory tree and group files with identical content. Compare file sizes first so large files are hashed only when sizes collide, then hash the rest. Be ready to discuss…
Last asked Sep 2026
Fix a function-cache key function, then make it durableCodingA small memoizing cache for arbitrary callables has a broken key function that mishandles positional versus keyword arguments and defaults. Fix the keying first, then extend the…
Last asked Sep 2026
Distributed mode and median across worker shardsCodingYou are given a cluster of k worker nodes, numbered 0 to k - 1. Each worker holds its own unsorted slice (shard) of a large integer dataset. A worker can read only its own shard…
Last asked Sep 2026
Repository Merkle tree hashing and change detectionCodingHash a code repository into a Merkle tree over files and directories so changed paths are found by comparing hashes top-down instead of rescanning everything. Reported on…
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
Deduplicate a 256 GB file with 128 MB of memoryCodingA 256 GB file holds records, some duplicated. The machine has 128 MB of memory. Produce output that holds every distinct record exactly once. Explain the approach (for example…
Last asked Sep 2026
First value seen twice in a left-to-right scanCodingWalk an integer array from left to right and return the first value you meet for the second time, or -1 when no value repeats. The answer is decided by where the second occurrence…
Last asked Sep 2026
First missing positiveCodingGiven an unsorted list of integers that may include negatives, zeros, duplicates and values much larger than the list's length, return the smallest positive integer that does not…
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
+6
Design TinyURLSystem designTurn long URLs into short links and redirect visitors fast, at billions of redirects a day, with optional custom aliases and expiry.
Last asked Sep 2026
Find robots by distances to the nearest blockerCodingYou are given an m x n board. Each cell is one of: - 'O': a robot - 'E': an empty cell - 'X': a blocker You are also given a query of four integers left, top, bottom, right. For a…
Last asked Sep 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
Remove the fewest distinct values to delete half the arrayCodingPick a set of distinct values and delete every occurrence of each from an integer array. Return the smallest set size that removes at least half of the elements. Explain the…
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
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
Most frequent IP addresses in a file too big for memoryCodingA log file of IP addresses is far larger than memory. Find the most frequent address, then the top K. Explain how you split the work into pieces that fit in memory, count each…
Last asked Jun 2026
Encode and decode TinyURLCodingImplement a class that turns a long URL into a short one and back, in memory. Discuss counter-based versus random or hash-based codes, collisions, the same long URL submitted…
Last asked Jun 2026
Stream of points: remove any pair within a distanceCodingPoints arrive one at a time. When a new point is within a given distance of a point already stored, remove that pair; otherwise store the new point. Keep each step fast as the…
Last asked Jun 2026
Largest axis-aligned rectangle from pointsCodingYou are given a list of distinct points on a 2D integer grid. Each point is a pair x, y. Return the largest area of a rectangle whose sides are parallel to the x and y axes and…
Last asked Apr 2026
Line reflection symmetry with duplicate pointsCodingGiven points in the plane, possibly with duplicates, decide whether there is a vertical line that reflects the set onto itself.
Last asked Feb 2026
Max points on a lineCodingGiven points on a plane, return the largest number of them that lie on one straight line. Explain how you represent slopes exactly (reduced fractions, not floats) and handle…
Last asked Feb 2026
Remove duplicates in place from an unsorted arrayCodingRemove repeated values from an unsorted array in place, keeping the first occurrence of each and their order, and return the new length. Compare a hash set with sorting first, and…
Last asked Feb 2026
Friend groups by matching last k moviesCodingA streaming service stores each customer's full viewing history, oldest title first. Two customers are friends when the set of their last k watched titles is the same. Order…
Last asked Jan 2026
Top k lottery winners by spendingCodingGiven lottery entries that record who bought tickets and how much they spent, return the top k people ranked by total spending. Discuss ties and how you would keep the answer…
Last asked Jan 2026
Design a hash map without built-in librariesCodingImplement a hash map with put, get and remove without using any built-in hash table. Choose the bucket layout, the hash function and how collisions are handled, and explain when…
Last asked Nov 2025
Stream deduplication with near-duplicate suppressionCodingPass through a stream of strings, keeping only the first copy of each. Then also drop strings that are near-duplicates of something already kept, above a similarity threshold, and…
Last asked Nov 2025
Products your friends bought that you haven'tCodingA user has a list of friends and a list of purchases; each purchase has a product id, an amount and a purchase id, and a product catalog maps product ids to names. Return the…
Last asked Oct 2025
Most popular concert city from votes, with name variants and weightsCodingGiven vote records naming venue cities for a concert, return the most popular city. Follow-ups: the same city appears under many spellings (NYC, New York, new york city), so how…
Last asked Sep 2025
Report a user's license usage from a log fileCodingWrite a command-line script called as ./script.sh <username <logfile. Each log line looks like timestamp license user@hostname. Print the result the interviewer asks for about…
Last asked Sep 2025
Sort characters by frequencyCodingGiven a string, rearrange it so the most frequent characters come first, each character's copies kept together. Count with a hash map, then sort or bucket by count.
Last asked Sep 2025
Unique visitors per department from click logsCodingEach product has an id, a department, a category and a subcategory; the last three form a parent-child hierarchy. A click log records (customerid, productid). For any department…
Last asked Sep 2025
Real-time top-k products from purchase eventsCodingPurchase events (customerid, itemid, timestamp) arrive continuously. Keep a structure that returns the k most purchased items at any moment. Discuss the cost of updates and…
Last asked Sep 2025
Most equal values after shifting one subarrayCodingGiven an array, you may once choose any contiguous subarray and add the same integer (positive or negative) to all of its elements. Return the largest number of equal values the…
Last asked Aug 2025
Find the number that appears an odd number of timesCodingIn an integer array exactly one value appears an odd number of times. Return it. A hash map is fine at first; the follow-up asks for O(1) extra space, which XOR gives.
Last asked Aug 2025
Valid Sudoku in one passCodingDecide whether a partly filled 9x9 Sudoku board is valid: no digit repeats in any row, column or 3x3 box (empty cells are ignored). Check it in a single pass over the board.
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
Top-k clicked items over a rolling windowCodingClick events arrive with timestamps. Keep the k most clicked items over the last fixed window of time, evicting old clicks as time moves on. Follow-ups asked about optimizing it…
Last asked Jul 2025
Most frequent user in access logsCodingEach log entry is a string holding a username and an access timestamp. Return the user who appears most often. Discuss ties, malformed lines and logs too big for memory.
Last asked Jul 2025
Inventory allocation with backordersCodingOrders arrive for products whose stock is limited. Allocate stock to orders as they come in; when a product runs out, record the unfilled part as a backorder and fill backorders…
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
Music player that plays the most frequent unplayed songCodingUsers keep arriving, each with a list of songs, and every song's count goes up each time it appears. The player must always play the most frequent song that has not been played…
Last asked Mar 2025
One-dimensional array with distinct elementsCodingSolve an array task constrained to distinct elements, using hashing to enforce uniqueness in linear time. Reported on 1point3acres' Glean bank; reconstructed in our own words.
Two-dimensional array with distinct elementsCodingProcess a two-dimensional array tracking distinct elements per row or column. Reported on 1point3acres' Glean bank; reconstructed in our own words.
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…
Count warehouse robots boxed in on all four sidesCodingA warehouse floor is modeled as an integer grid. Each delivery robot sits at a point (xi, yi), and no two robots share a point. A robot is idle when there is at least one other…
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…
Route a request along diagonal server redirectsCodingServers sit at distinct integer points on an infinite 2D grid. locationsi = x, y is the position of server i. A request starts at server 0, which counts as visited. You are given…
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. -…