Algorithm practice problems

Laddered easy, medium and hard coding problems for every algorithms topic, graded in your browser against hidden tests. Hints and Python + C++ solutions.

Arrays & strings

• Two Sum (Easy) — Find the two indices whose values add up to a target.
• Running Sum (Easy) — Build the array of prefix sums.
• Move Zeroes (Easy) — Push all zeroes to the end, keeping the order of the rest.
• Valid Palindrome (Easy) — Ignore case and non-alphanumerics — is it a palindrome?
• Maximum Subarray (Medium) — Largest sum of any contiguous subarray (Kadane's algorithm).
• Product of Array Except Self (Medium) — result[i] = product of every element except nums[i] — no division.
• Longest Substring Without Repeating (Medium) — Sliding window over a string with a last-seen map.
• Trapping Rain Water (Hard) — How much water is trapped between the bars after it rains?
• Group Anagrams (Hard) — Bucket words by their sorted-letter signature.

Searching

• Binary Search (Easy) — Find a target in a sorted array in O(log n).
• Search Insert Position (Easy) — Where would the target go in a sorted array?
• Search in Rotated Sorted Array (Medium) — Binary search in a sorted array that was rotated at an unknown pivot.
• Median of Two Sorted Arrays (Hard) — Find the median of two sorted arrays as a single value.

Recursion

• Factorial (Easy) — Compute n! with a base case and a recursive step.
• Fibonacci Number (Easy) — Return the n-th Fibonacci number (0-indexed).
• Sum of Digits (Easy) — Add up the decimal digits of a non-negative integer.
• Generate Parentheses (Medium) — All well-formed combinations of n pairs of parentheses.
• Permutations (Hard) — Every ordering of a list of distinct integers.

Hashing

• Contains Duplicate (Easy) — Does any value appear at least twice?
• Valid Anagram (Easy) — Is t a rearrangement of the letters of s?
• First Unique Character (Easy) — Index of the first non-repeating character.
• Top K Frequent Elements (Medium) — Return the k most frequent values (any order).
• Longest Consecutive Sequence (Hard) — Length of the longest run of consecutive integers — in O(n).

Stacks & queues

• Valid Parentheses (Easy) — Are the brackets balanced and correctly nested?
• Reverse String (Easy) — Return the characters of a string in reverse order.
• Daily Temperatures (Medium) — Days until a warmer temperature — a monotonic stack.
• Largest Rectangle in Histogram (Hard) — Biggest axis-aligned rectangle fitting under a histogram.

Sorting

• Merge Two Sorted Arrays (Easy) — The merge step at the heart of merge sort.
• Kth Largest Element (Medium) — Find the k-th largest value (1-indexed) in an unsorted array.
• Count Inversions (Hard) — How far is the array from sorted? Count out-of-order pairs in O(n log n).

Trees

• BST Search Path (Easy) — Trace the nodes visited while searching a BST.
• Kth Smallest in a BST (Medium) — An in-order walk of a BST yields sorted order.
• Lowest Common Ancestor in a BST (Hard) — Use the BST ordering to find the split point of two values.

Traversals

• Inorder Traversal (Easy) — Visit left, node, right — return the value sequence.
• Level-Order Traversal (Medium) — Group node values by depth using a BFS queue.
• Zigzag Level-Order (Hard) — Level-order, but alternate left→right and right→left per level.

Linked lists

• Reverse a Linked List (Easy) — Flip the order of the nodes.
• Remove Nth Node From End (Medium) — Delete the n-th node counting from the tail in one pass.
• Add Two Numbers (Linked Lists) (Hard) — Add two numbers stored as digit lists, least-significant first.

Pathfinding

• Number of Islands (Easy) — Count connected groups of land in a grid.
• Shortest Path in a Grid (Medium) — Fewest cells from top-left to bottom-right (BFS finds it).
• Rotting Oranges (Hard) — Multi-source BFS: how long until every fresh orange rots?

Heaps

• Last Stone Weight (Easy) — Repeatedly smash the two heaviest stones — a max-heap.
• K Smallest Elements (Medium) — Return the k smallest values, sorted ascending.
• Merge K Sorted Lists (Hard) — Merge many sorted lists with a min-heap of their heads.

Dynamic programming

• Climbing Stairs (Easy) — How many ways to climb n stairs taking 1 or 2 steps? (1D DP)
• Coin Change (Medium) — Fewest coins to make an amount (1D DP over amounts).
• Longest Common Subsequence (Hard) — Length of the longest subsequence shared by two strings (2D DP).
• Assignment Problem (Bitmask DP) (Hard) — Assign n workers to n tasks at minimum cost — dp over subsets.
• Travelling Salesman (Held-Karp) (Hard) — Shortest tour visiting every city once — exact, via bitmask DP.

Bit manipulation

• Single Number (Easy) — Every value appears twice except one — find it with XOR.
• Counting Bits (Medium) — Popcount for every number 0..n in O(n) using a bit DP.

Math for algorithms

• Greatest Common Divisor (Easy) — Euclid's algorithm: gcd(a, b) = gcd(b, a mod b).
• Count Primes (Medium) — How many primes are strictly less than n? — the sieve.
• Modular Exponentiation (Medium) — Compute base^exp mod m fast with binary exponentiation.
• Binomial Coefficient mod p (Hard) — C(n, r) mod a prime, using factorials and a modular inverse.

Greedy algorithms

• Activity Selection (Easy) — Max non-overlapping intervals — take earliest finish each time.
• Jump Game (Medium) — Can you reach the last index? Track the furthest reachable point.
• Minimum Arrows to Burst Balloons (Hard) — Fewest points that stab every interval — a greedy sweep.
• Maximum Product Subarray (Hard) — Largest product of a contiguous subarray — track max AND min.

Strings & pattern matching

• Find the Index of First Match (Easy) — Return the first index where `needle` occurs in `haystack`.
• Count Pattern Occurrences (Medium) — How many times does the pattern occur (overlaps allowed)?
• Longest Happy Prefix (Hard) — Longest proper prefix that is also a suffix — the KMP failure value.

Graphs

• Number of Connected Components (Easy) — Count the connected pieces of an undirected graph.
• Course Schedule (Medium) — Can you finish all courses? — detect a cycle in a dependency graph.
• Topological Order (lexicographically smallest) (Hard) — Order a DAG so every edge points forward — smallest order, or none.

Divide & conquer

• Fast Power (xⁿ) (Easy) — Compute xⁿ in O(log n) by squaring.
• Majority Element (Medium) — The element appearing more than n/2 times.

Game theory

• Nim — Who Wins? (Easy) — First player wins iff the XOR of pile sizes is non-zero.
• Subtraction Game (Medium) — Win/Lose DP: can the first player win from n with allowed moves?
• Grundy Number (Sprague–Grundy) (Hard) — Compute the Grundy value of a subtraction-game position.

Minimum spanning trees

• Valid Spanning Tree (Easy) — Check whether undirected edges connect all n nodes without a cycle.
• Minimum Connection Cost (Medium) — Return the MST weight of a weighted undirected graph, or -1 if disconnected.
• Second Best MST (Hard) — Find the cheapest spanning tree whose total weight is strictly larger than the MST.