Code Room
Practice the whole technical round. Coding, system design, code review, and on-call — drilled by level, role, and subject, run against tests in the browser, and graded by a coach, not a judge.
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Coding Write, debug, and run code against tests — narrated. System design Design an architecture, clarify, scale, trade off. Code review Read a diff — find the bug, the smell, the risk. On-call Diagnose an incident, mitigate, write the postmortem. Vibe coding Direct an AI to build it — then catch what it got wrong.
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Show more Given an array of integers and a target, return the indices of the two numbers that add up to the target. Assume exactly one solution and you may not use the same element twice.
cod-p001 Arrays strings · Entry–Mid · ~15 min · Algorithms & data structures
Given a string, return the index of the first character that does not repeat anywhere in the string. Return -1 if every character repeats.
cod-p002 Hashing · Entry–Mid · ~15 min · Algorithms & data structures
Given a list of intervals [start, end], merge all overlapping intervals and return the result sorted by start.
cod-p003 Intervals · Mid–Senior · ~25 min · Algorithms & data structures
Given a grid of 1s (land) and 0s (water), count the number of islands. An island is a group of 1s connected horizontally or vertically.
cod-p004 Graphs · Mid–Senior · ~30 min · Algorithms & data structures
Given an array of integers, return the length of the longest strictly increasing subsequence.
cod-p005 Dynamic programming · Senior–Staff · ~35 min · Algorithms & data structures
You are given a directed network of n nodes (labelled 1..n) and a list of travel times [u, v, w] meaning a signal from u reaches v after w time. A signal starts at node k. Return…
cod-p006 Graphs · Senior–Staff · ~35 min · Algorithms & data structures
Given a string, determine whether it is a palindrome, considering only alphanumeric characters and ignoring case.
cod-p007 Arrays strings · Entry–Mid · ~15 min · Algorithms & data structures
Given an integer array, return the largest sum of any contiguous subarray (at least one element).
cod-p008 Dynamic programming · Entry–Mid · ~15 min · Algorithms & data structures
Given a string of brackets '()[]{}', determine whether every bracket is closed by the same type in the correct order.
cod-p009 Stack · Mid–Senior · ~25 min · Algorithms & data structures
Given a sorted array that has been rotated at an unknown pivot and a target, return the target's index or -1.
cod-p010 Binary search · Mid–Senior · ~25 min · Algorithms & data structures
Given a string, return the length of the longest substring without repeating characters.
cod-p011 Sliding window · Mid–Senior · ~25 min · Algorithms & data structures
Given an array of heights, pick two lines that together with the x-axis form a container holding the most water. Return that maximum area.
cod-p012 Two pointers · Mid–Senior · ~25 min · Algorithms & data structures
Given coin denominations and a target amount, return the fewest coins that sum to the amount, or -1 if it cannot be made.
cod-p013 Dynamic programming · Mid–Senior · ~25 min · Algorithms & data structures
Given a set of intervals, return the minimum number you must remove so that the rest are non-overlapping.
cod-p014 Intervals · Mid–Senior · ~25 min · Algorithms & data structures
Given two strings, return the minimum number of single-character insertions, deletions, or substitutions to turn the first into the second.
cod-p015 Dynamic programming · Senior–Staff · ~35 min · Algorithms & data structures
Given a string and a dictionary of words, return whether the string can be segmented into a space-separated sequence of dictionary words.
cod-p016 Dynamic programming · Senior–Staff · ~35 min · Algorithms & data structures
There are numCourses courses (0..numCourses-1) and prerequisite pairs [a, b] meaning b must come before a. Return whether all courses can be finished.
cod-p017 Graphs · Senior–Staff · ~35 min · Algorithms & data structures
Return the kth largest element in an unsorted array (the kth in sorted order, not the kth distinct).
cod-p018 Heap · Senior–Staff · ~35 min · Algorithms & data structures
Given a string, return the length of its longest palindromic subsequence (characters need not be contiguous).
cod-p019 Dynamic programming · Senior–Staff · ~35 min · Algorithms & data structures
A teammate wrote this helper to report whether a list contains any duplicate value, but it always returns the wrong answer. Find and fix the bug.
cod-p020 Hashing · Entry–Mid · ~15 min · Algorithms & data structures
This binary search should return the index of target in a sorted array, or -1, but it misses the target in some cases. Find and fix the bug.
cod-p021 Binary search · Mid–Senior · ~25 min · Algorithms & data structures
Move all zeroes in an integer array to the end while keeping the relative order of the non-zero elements. Return the result.
cod-p022 Arrays strings · Entry–Mid · ~15 min · Algorithms & data structures
Given an integer array, return an array where each position holds the product of all other elements. You may not use division.
cod-p023 Arrays strings · Mid–Senior · ~25 min · Algorithms & data structures
Return the element that appears more than n/2 times in the array (guaranteed to exist).
cod-p024 Arrays strings · Entry–Mid · ~15 min · Algorithms & data structures
Rotate an array to the right by k steps and return the result. k may exceed the array length.
cod-p025 Arrays strings · Mid–Senior · ~25 min · Algorithms & data structures
Given daily prices, return the maximum profit from a single buy-then-sell. Return 0 if no profit is possible.
cod-p026 Dynamic programming · Entry–Mid · ~15 min · Algorithms & data structures
Given daily prices, return the maximum profit when you may buy and sell as many times as you like (one share at a time).
cod-p027 Greedy · Entry–Mid · ~15 min · Algorithms & data structures
You climb a staircase of n steps, taking 1 or 2 steps at a time. Return the number of distinct ways to reach the top.
cod-p028 Dynamic programming · Entry–Mid · ~15 min · Algorithms & data structures
Given house values along a street, return the maximum you can rob without robbing two adjacent houses.
cod-p029 Dynamic programming · Mid–Senior · ~25 min · Algorithms & data structures
Houses are arranged in a circle (first and last are adjacent). Return the max you can rob without robbing two adjacent houses.
cod-p030 Dynamic programming · Mid–Senior · ~25 min · Algorithms & data structures
A robot at the top-left of an m x n grid can only move right or down. Return the number of distinct paths to the bottom-right.
cod-p031 Dynamic programming · Mid–Senior · ~25 min · Algorithms & data structures
Given an m x n grid of non-negative numbers, return the minimum path sum from top-left to bottom-right, moving only right or down.
cod-p032 Dynamic programming · Mid–Senior · ~25 min · Algorithms & data structures
Given an m x n matrix, return all its elements in spiral order (clockwise from the top-left).
cod-p033 Matrix · Mid–Senior · ~25 min · Algorithms & data structures
Given two strings, return whether the second is an anagram of the first.
cod-p034 Hashing · Entry–Mid · ~15 min · Algorithms & data structures
Return whether the first string is a subsequence of the second (characters in order, not necessarily contiguous).
cod-p035 Two pointers · Entry–Mid · ~15 min · Algorithms & data structures
Given two strings, return the length of their longest common subsequence.
cod-p036 Dynamic programming · Mid–Senior · ~25 min · Algorithms & data structures
Given a binary matrix of 0s and 1s, return the area of the largest square containing only 1s.
cod-p037 Dynamic programming · Senior–Staff · ~35 min · Algorithms & data structures
Return the number of set bits (1s) in the binary representation of a non-negative integer.
cod-p038 Bit manipulation · Entry–Mid · ~15 min · Algorithms & data structures
Given n, return an array where index i holds the number of 1 bits in i, for all i from 0 to n.
cod-p039 Bit manipulation · Mid–Senior · ~25 min · Algorithms & data structures
An array contains n distinct numbers taken from 0..n. Return the single number that is missing.
cod-p040 Math · Entry–Mid · ~15 min · Algorithms & data structures
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