Coding & Algorithms Interview Questions
Practice 3,469 real Coding & Algorithms interview questions for 2026. Covers companies like Meta, Amazon, Google, Uber, and Microsoft. Real questions from actual interviews with detailed solutions — ideal for focused interview preparation. Coding & Algorithms interview questions are the single most common technical gate for software engineering roles and are used to evaluate problem framing, algorithm selection, complexity trade-offs, correctness, edge-case reasoning, and clear communication under time pressure. Expect this category to appear in early technical screens and repeatedly across onsite loops at companies that weight it heavily (notably Meta, Google, and Amazon). Interviews will test arrays and strings, trees and graphs, dynamic programming, hashing and two‑pointer techniques, and the candidate’s ability to test and optimize code in a shared editor. Prepare with timed mock interviews, practice in the same editor you’ll use live, and build a progressive schedule that moves from fundamentals to medium/hard problems; typical candidates invest several weeks to a few months of focused practice depending on seniority and target company.
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Find smallest N for 5-and-21 sums
You are given two positive integers: 5 and 21. We say a positive integer x is representable if it can be written in the form \[ x = 5a + 21b \] for so...
Implement delimiter-free string codec
Design an encoder/decoder for a list of strings. - Implement two functions: - encode(strings: List[str]) -> str - decode(blob: str) -> List[str] -...
Optimize repeated-value vectors and compute exclusive times
You are given two separate coding tasks from an interview. Task 1: Optimize storage for vectors with repeated values, then compute dot product You are...
Implement several string, tree, and BFS problems
This multi-part problem evaluates proficiency with core data structures and algorithms: binary tree traversal with column-aligned output, string parsi...
Merge multiple sorted arrays using min-heap
Problem You are given k sorted arrays of integers: - arrays[0], arrays[1], ..., arrays[k-1] - Each arrays[i] is sorted in non-decreasing order. - The ...
Reverse linked list in fixed-size groups
Problem You are given the head of a singly linked list and an integer k (k >= 1). Reverse the nodes of the list in-place in groups of size k: - The no...
Compute outer boundary of an N-ary tree
You are given the root of a rooted N-ary tree. Each node contains: - An integer value. - An ordered list of children from left to right (0 or more chi...
Match payments to invoices by memo or amount
Scenario You are building a small reconciliation tool that matches payments to invoices. Data structures Assume you are given: - invoices: a list of i...
Find all unique triplets summing to zero
Problem Given an integer array nums, return all unique triplets [nums[i], nums[j], nums[k]] such that: - i, j, and k are distinct indices (i != j != k...
Compute win probability in coin-toss game
Two players, A and B, play the following game with a fair coin: - They toss the coin alternately: A tosses first, then B, then A, then B, and so on. -...
Count Regions in a Binary Grid
This question evaluates understanding of grid-as-graph representations, identification of connected components, and competency in algorithmic problem-...
Validate parentheses with one or three bracket types
Problem Given a string s consisting only of bracket characters, determine whether it is valid. A string is valid if: - Every opening bracket has a cor...
Match payments to invoices by memo or amount
You are building a small payment-to-invoice matching utility. Data You are given: - invoices: a list of invoice records with: - invoice_id (string) ...
Implement transaction network queries
Problem You are building a small in-memory library to track pairwise customer transactions and answer connectivity questions about who has transacted ...
Count islands using BFS without modifying grid
You are given an m x n binary grid grid where: - 1 represents land - 0 represents water - Islands are groups of horizontally or vertically adjacent la...
Print directory tree with indentation
You are given a root directory of a filesystem represented as a tree. Each node is either a directory (can have children) or a file (no children). Pri...
Minimize time using elevator then climb stairs
This question evaluates a candidate's ability to model and optimize a discrete decision under resource constraints, testing algorithmic optimization, ...
Find all cells reachable by downhill flow
Problem You are given an m x n grid heights, where heights[r][c] is the elevation of cell (r, c). A drop of water is placed at a starting cell start =...
Implement hierarchical folder access check
You are asked to simulate a simple hierarchical file system and implement an access-check function. The file system consists of folders arranged in a ...
Compute max coins with 3-step token moves
You are given a one-dimensional board with n positions represented by a string s of length n: - . = empty cell - T = token - C = coin You may move tok...