Coding & Algorithms Interview Questions
Practice 3,013 real Coding & Algorithms interview questions for 2026. Coding & Algorithms interview questions for top tech firms like Meta, Amazon, Google, Uber, and TikTok — real questions from actual interviews with detailed solutions — to power your interview preparation. This collection is sharply focused on algorithmic patterns (arrays, strings, sliding windows, trees/graphs, dynamic programming, greedy, two-pointers, and hashing), clean, review-ready code, and the explanation skills that distinguish hires. In 2026 expect familiar medium-to-hard problems plus newer twists: AI-enabled or multi-file coding rounds at some companies, more emphasis on explainability and edge-case reasoning, and occasional real-world performance constraints (streaming, concurrency, memory limits). Interviewers evaluate problem framing, algorithm choice, complexity trade-offs, test-case thinking, and communication under time pressure. Best prep mixes pattern-based practice, timed mock interviews, code-review style polishing, and rehearsed explanations of complexity and tradeoffs. Focus your last-mile prep on writing correct, readable code quickly and on explaining why your solution is robust and efficient for production scenarios.
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Count decodings of a numeric string
Given a string s of digits representing an encoded message where '1' maps to 'A', ..., '26' maps to 'Z', and '0' cannot appear alone (it must be part ...
Plan bicycle routes on a city map
You are given a city bicycle network as a weighted graph where edges encode distance, bike-lane availability, elevation gain, and traffic risk. Comput...
Implement file deduplication at scale
Write a program to deduplicate files in a very large directory tree. Identify groups of identical files without loading entire files into memory. Outl...
Count connected delivery zones
Given an m x n grid representing a service area, each cell is either 'Z' (deliverable zone) or '#' (blocked). Two 'Z' cells belong to the same zone if...
Count subarrays with product under target
Given an array of positive integers nums and an integer T > 1, return the number of contiguous subarrays whose product is strictly less than T. Provid...
Navigate unknown grid to locate target
You control a mouse in an unknown 2D maze. You do not know the maze dimensions or your absolute coordinates. The interface exposes move() → bool (atte...
Solve max-subarray, min-unique, and grid-jump
1) Given an integer array nums, find a contiguous subarray with the maximum possible sum. Return (maxSum, startIndex, endIndex). If multiple subarrays...
Design a parser for a hypothetical language
You are given a set of syntax rules for a hypothetical programming language written in BNF/EBNF. Design data structures and algorithms that: ( 1) pars...
Implement prioritized refund allocation engine
Implement a refund allocation function that takes: (a) a list of payments, each with a unique paymentId, method ∈ {CREDIT, CREDIT_CARD, PAYPAL}, ISO-8...
Solve six algorithmic problems
Answer the following independent algorithmic prompts. For each, explain your approach, justify data structures, analyze time/space complexity, and pro...
Clone list with random pointers
Question You are given the head of a singly linked list where each node has two pointers: next and random. The random pointer may be null or point to ...
Maximize one stock trade profit
You're given an array prices of length n where prices[i] is the stock price on day i. You may complete at most one transaction (buy once, then sell la...
Implement multi-network card validator with Luhn
Design and implement a payment card validation module that supports multiple networks and error scenarios. Requirements: 1) Luhn checksum - Implement ...
Implement a same-host web crawler
Implement a web crawler that, given a starting URL and an interface get_links(url) -> Iterable[str], discovers all pages under the same hostname. Requ...
Implement binary search for boundary index
Given a sorted integer array nums (length n) and an integer target t, return the index of the first element in nums that is greater than or equal to t...
Implement image carousel with comments and upvotes
Implement an image carousel that supports: 1) cycling forward/backward and autoplay with a configurable interval; 2) a per-image comment thread with c...
Determine allow/deny for an IP via CIDR rules
Problem You are implementing a simple IPv4 firewall. You are given an ordered list of rules. Each rule has: - an action: either ALLOW or DENY - a CIDR...
Minimize run-length encoded length with deletions
Problem You are given a string s (uppercase English letters) and an integer k. You may delete at most k characters from s. After deletions, you compre...
Find the longest palindromic substring
Problem Given a string s, return the longest contiguous substring of s that is a palindrome (reads the same forward and backward). Input - s: a non-em...
Design O(1) insert/delete and frequency-weighted random
Problem Design a data structure that supports the following operations in average \(O(1)\) time: 1. add(x) -> bool - Inserts value x into the colle...