Fibonacci(n)
Return the nth Fibonacci number efficiently.
Flatten nested list
Yield every integer from an arbitrarily nested list, depth-first.
Hash map merge
Write a function that merges two dictionaries recursively, combining values and preserving structure.
Quicksort
Implement quicksort and return a sorted list.
Graph DFS
Return all nodes reachable from a start node via DFS.
Merge Sort
Implement merge_sort(numbers) that returns a sorted copy of the input list using the merge sort algorithm.
Strobogrammatic number II
Given a positive integer n, return all strobogrammatic numbers of length n in ascending order.
Lowest Common Ancestor in a Binary Tree
Implement a function to find the lowest common ancestor (LCA) of two nodes in a binary tree.
Sum Root to Leaf Numbers
Given the root of a binary tree, compute the total sum of all root-to-leaf numbers.
Delete Node in BST
Implement a function that deletes a key from a binary search tree and returns the new root.
Trim BST to range
Implement a function to trim a BST to only retain nodes with values in a given inclusive range.
Validate BST
Implement a function to determine if a binary tree is a valid Binary Search Tree.
Kth Smallest in BST
Implement a function that returns the k-th smallest value in a binary search tree.
Count Complete Tree Nodes
Given a complete binary tree, return its node count faster than O(n) time.
Combination Generator
Write a recursive function that returns all combinations of length k from a list of distinct integers.
Generate Parentheses
Given n, generate all valid strings of n pairs of parentheses.
Letter Combinations of a Phone Number
Given a string of digits, return all possible letter combinations that the number could represent on a phone keypad.
Subsets
Return all subsets (the power set) of a list of distinct integers.
Subsets II
Given a collection of integers that might contain duplicates, return all possible unique subsets.
N-Queens Count
Count the number of distinct valid placements of n non-attacking queens on an n×n chessboard.
Word Search Backtrack
Determine if a given word exists in a 2D board by tracing adjacent cells without reusing any cell.
Palindrome Partitioning
Given a string s, return all possible palindrome partitioning of s.
Partition Equal Subset Sum (Backtracking)
Write a function that uses backtracking to decide if a list of positive integers can be partitioned into two subsets with equal sum.
Combination Sum
Given a list of distinct positive integers and a target, return all unique combinations that sum to the target.
Showing 1–24 of 28 challenges · medium
Guide: free Python coding challenges
Practice Python by solving problems
PythonSkillset challenges are hands-on coding exercises from beginner to advanced. Open a challenge, read the problem, write Python in the split-pane editor, and run tests with Pyodide — no install required.
How to use the arena
- Pick a category — basics, algorithms, strings, and more
- Open a challenge, read the statement, and edit the starter code
- Run tests, fix failures, then try a related quiz or tutorial lesson
Challenges vs tutorials and quizzes
Challenges test what you can build under constraints. For guided teaching, use our Python tutorials. For quick checks, try quizzes or copy snippets from code samples.