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.

Built for

Learn conceptsEvery concept, animated

CodingMedium Runnable
#65Count distinct people from account merges
There are n accounts labeled 0..n-1. You are given a list of merge operations, each [a, b], meaning accounts a and b are declared to belong to the same person. After processing…
cod-g005Union find·Mid–Senior·~25 min·Algorithms & data structures
CodingEasy Runnable
#1564First non-repeating character
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-p002Hashing·Entry–Mid·~15 min·Algorithms & data structures
CodingEasy Runnable
#1570Maximum subarray sum
Given an integer array, return the largest sum of any contiguous subarray (at least one element).
cod-p008Dynamic programming·Entry–Mid·~15 min·Algorithms & data structures
CodingMedium Runnable
#1571Valid parentheses
Given a string of brackets '()[]{}', determine whether every bracket is closed by the same type in the correct order.
cod-p009Stack·Mid–Senior·~25 min·Algorithms & data structures
CodingMedium Runnable
#1600Counting bits
Given n, return an array where index i holds the number of 1 bits in i, for all i from 0 to n.
cod-p039Bit manipulation·Mid–Senior·~25 min·Algorithms & data structures
CodingMedium Runnable
#1606Jump game
Each value is the max jump length from that index. Return whether you can reach the last index starting from index 0.
cod-p045Greedy·Mid–Senior·~25 min·Algorithms & data structures
CodingMedium Runnable
#800Implement fixed window counter limiter
Implement a per-key fixed-window counter rate limiter. Given a list of (key, ts) requests in non-decreasing ts order (integer seconds), a 'limit', and a 'window' size in seconds,…
cod-g226Rate limiter design·Mid–Senior·~30 min·Networking & APIs
CodingMedium Runnable
#64Lexicographically smallest task order
You are given n tasks labeled 0..n-1 and a list of prerequisite pairs [a, b] meaning task b must be completed before task a. Return the lexicographically smallest valid ordering…
cod-g004Topological sort·Mid–Senior·~30 min·Algorithms & data structures
CodingHard Runnable
#73Longest path in DAG
You are given a DAG of n nodes (0..n-1) with weighted directed edges [u, v, w], w >= 0, representing task durations along dependencies. Return the length of the longest path…
cod-g013Topological sort·Senior–Staff·~35 min·Algorithms & data structures
CodingHard Runnable
#848Probability of final score within target
You start with 0 points. While your score is strictly less than 'stop', you draw a number uniformly from 1..maxPts and add it. You stop drawing once your score is >= stop. Return…
cod-g277Probability dp·Senior–Staff·~40 min·Algorithms & data structures
CodingHard Runnable
#850Evaluate division equations
You maintain n variables and process a list of operations. Each operation is either ["set", a, b, w] meaning the ratio value(a)/value(b) equals the float w, or ["q", a, b] meaning…
cod-g279Union find·Senior–Staff·~35 min·Algorithms & data structures
CodingMedium Runnable
#1565Merge overlapping intervals
Given a list of intervals [start, end], merge all overlapping intervals and return the result sorted by start.
cod-p003Intervals·Mid–Senior·~25 min·Algorithms & data structures
CodingEasy Runnable
#1636Digital root
Repeatedly sum a non-negative integer's digits until a single digit remains; return it (the digital root).
cod-p075Math·Entry–Mid·~15 min·Algorithms & data structures
CodingMedium Runnable
#90Count square submatrices with all ones
Given an m x n binary matrix (entries are 0 or 1), count how many square submatrices have all entries equal to 1. Squares of every side length count, and squares overlapping each…
cod-g033Grid dp·Mid–Senior·~20 min·Algorithms & data structures
CodingHard Runnable
#97Partition to k equal sum subsets
Given an integer array nums and an integer k, determine whether it is possible to partition nums into exactly k non-empty subsets whose sums are all equal. Return true or false.…
cod-g040Bitmask dp·Senior–Staff·~30 min·Distributed systems, Algorithms & data structures
CodingHard Runnable
#107Maximum path sum in binary tree
Given a binary tree as a level-order array `arr` (using `null` for missing children, so JSON null arrives as Python None), find the maximum path sum. A path is any sequence of…
cod-g050Binary trees·Senior–Staff·~30 min·Algorithms & data structures
CodingMedium Runnable
#110Kth smallest element in BST
Given a binary search tree as a level-order array `arr` (null for missing) and a 1-indexed integer `k`, return the value of the k-th smallest element. The tree is a valid BST with…
cod-g053Binary search tree·Mid–Senior·~20 min·Algorithms & data structures
CodingMedium Runnable
#120Zigzag level-order traversal
Given a binary tree as a level-order array `arr` (null for missing), return its zigzag (boustrophedon) level-order traversal: the first level left to right, the second right to…
cod-g063Tree traversal·Mid–Senior·~20 min·Algorithms & data structures
CodingMedium Runnable
#123Distance between two nodes in binary tree
Given a general binary tree as a level-order array `arr` (null for missing) and two distinct-or-equal values `a` and `b` that are both present, return the number of edges on the…
cod-g066Lowest common ancestor·Mid–Senior·~25 min·Algorithms & data structures
CodingMedium Runnable
#899Wiggle sort array
Reorder the integer array nums IN PLACE so it forms a wiggle pattern: nums[0] <= nums[1] >= nums[2] <= nums[3] >= ... Return the rearranged array. Many valid answers exist; the…
cod-g328Partitioning·Mid–Senior·~24 min·Distributed systems, Algorithms & data structures
CodingMedium Runnable
#140Largest number from concatenation
Given a list of non-negative integers nums, arrange them so that concatenating their decimal representations forms the largest possible number, and return that number as a string.…
cod-g083Sorting·Mid–Senior·~25 min·Algorithms & data structures
CodingMedium Runnable
#145Palindrome with at most one deletion
Given a lowercase string s, return True if it can be made into a palindrome by deleting at most one character (deleting zero characters is allowed, so any palindrome qualifies).…
cod-g088Two pointers·Mid–Senior·~20 min·Algorithms & data structures
CodingMedium Runnable
#148Shortest subarray to sort
Given an integer array a, find the length of the shortest contiguous subarray that, if sorted in place, would make the entire array sorted in ascending order. If the array is…
cod-g091Sorting·Mid–Senior·~25 min·Algorithms & data structures
CodingMedium Runnable
#201Binary tree right-side view
A binary tree is given in level-order array form where null children are represented by the value None... but to keep inputs JSON-friendly, it is given as a flat list using -1 as…
cod-g104Queues·Mid–Senior·~25 min·Algorithms & data structures
CodingHard Runnable
#949Decode string with wildcards
A message of digits is encoded with 'A'->1 ... 'Z'->26. The string may also contain '*', a wildcard for any digit 1-9. Given such a string s (1 <= len(s) <= 10^5), return the…
cod-g380Strings·Senior–Staff·~30 min·Algorithms & data structures
CodingMedium Runnable
#1566Number of islands
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-p004Graphs·Mid–Senior·~30 min·Algorithms & data structures
CodingMedium Runnable
#627Find the smallest string period
The smallest period of a string s is the length of the shortest prefix p such that s is formed by repeating p a whole number of times (so "abcabcabc" has smallest period 3, "aaaa"…
cod-g143Kmp·Mid–Senior·~25 min·Algorithms & data structures
CodingHard Runnable
#649Longest duplicate substring
Given a string s, return the length of the longest substring that appears at least twice in s (the two occurrences may overlap). If no character repeats in a way that yields a…
cod-g145Rolling hash·Senior–Staff·~35 min·Algorithms & data structures
CodingMedium Runnable
#660Count matching Z-algorithm indices
For a string s, consider each starting index i (including 0). Index i counts if the substring of s that begins at i shares at least one leading character with s itself —…
cod-g146Z algorithm·Mid–Senior·~25 min·Algorithms & data structures
CodingMedium Runnable
#671Substring search with Z-algorithm
Implement substring search: return the index of the first occurrence of pat in text, or -1 if it does not occur. If pat is empty, return 0. Solve it in linear time using the…
cod-g147Z algorithm·Mid–Senior·~25 min·Algorithms & data structures
CodingMedium Runnable
#693One edit distance check
Given two strings a and b, return True if they are at most one edit apart, where an edit is inserting, deleting, or replacing a single character (zero edits, i.e. equal strings,…
cod-g149Edit distance·Mid–Senior·~20 min·Algorithms & data structures
CodingMedium Runnable
#704Count palindromic substrings
Return the number of palindromic substrings of s, counting substrings at different positions separately even if they are equal as strings. Single characters count, so "aaa" has 6…
cod-g150Palindromes·Mid–Senior·~20 min·Algorithms & data structures
CodingMedium Runnable
#728Evaluate arithmetic expression
Evaluate a basic arithmetic expression string containing non-negative integers and the binary operators +, -, *, / separated by the operators (and possibly spaces). There are no…
cod-g153String parsing·Mid–Senior·~30 min·Algorithms & data structures
CodingMedium Runnable
#729Decode nested bracket encoding
Decode a string encoded with the rule k[encoded], meaning the bracketed substring is repeated k times; encodings can be nested. k is a positive integer (possibly multiple digits)…
cod-g154String parsing·Mid–Senior·~25 min·Algorithms & data structures
CodingHard Runnable
#730Minimum window substring
Given strings s and t, return the shortest contiguous substring of s that contains every character of t including multiplicities (e.g. if t has two 'a's, the window must too). If…
cod-g155Strings·Senior–Staff·~35 min·Algorithms & data structures
CodingHard Runnable
#1567Longest increasing subsequence
Given an array of integers, return the length of the longest strictly increasing subsequence.
cod-p005Dynamic programming·Senior–Staff·~35 min·Algorithms & data structures
CodingHard Runnable
#731Minimum insertions for palindrome
Return the minimum number of characters you must insert anywhere into s to make it a palindrome. For example "mbadm" needs 2 insertions (one valid result is "mdbabdm"), while…
cod-g156Palindromes·Senior–Staff·~35 min·Algorithms & data structures
CodingHard Runnable
#732Shortest palindrome by prepending
You may only PREPEND characters to the front of s. Return the shortest palindrome obtainable this way. For example "aacecaaa" becomes "aaacecaaa" and "abcd" becomes "dcbabcd". An…
cod-g157Kmp·Senior–Staff·~35 min·Algorithms & data structures
CodingMedium Runnable
#851Minimum spanning tree weight
Given n nodes labeled 0..n-1 and a list of weighted undirected edges [u, v, w], return the total weight of a minimum spanning tree. When several edges have equal weight the choice…
cod-g280Mst·Mid–Senior·~25 min·Algorithms & data structures
CodingMedium Runnable
#733Validate IPv4 or IPv6 address
Given a string query, classify it as a valid "IPv4" address, a valid "IPv6" address, or "Neither". An IPv4 address is four dot-separated decimal groups, each 0-255 with no leading…
cod-g158String parsing·Mid–Senior·~25 min·Algorithms & data structures