Count Nice Subarrays
Given an array of integers, count the number of contiguous subarrays that contain exactly k odd numbers.
Reverse Pairs Count
Implement a function to count reverse pairs in an array efficiently.
Three Sum
Given an array of integers, return all unique triplets that sum to zero.
Closest Three Sum
Given an array of integers and a target, return the sum of three numbers that is closest to the target.
Interval List Intersections
Given two lists of sorted, disjoint intervals, return the list of intersections between them.
Minimum Size Subarray Sum
Given an array of positive integers, return the minimal length of a contiguous subarray with sum at least target, or 0 if none exists.
Fruit into Baskets
Given an array of integers representing fruit types, return the maximum number of fruits you can collect in a contiguous subarray with at most two distinct types.
Binary Subarray with Sum
Given a binary list and a goal sum, count the number of subarrays that add up to that goal.
Subarray product less than K
Count contiguous subarrays where the product of all elements is strictly less than a target K.
Longest Substring with At Most K Distinct Characters
Given a string and an integer k, find the length of the longest substring that contains at most k distinct characters.
Boats to Save People – Two Pointer
Implement a function that returns the minimum number of boats needed to rescue everyone, given a weight limit and each boat carrying at most two people.
Trapping Rain Water (Two Pointers)
Given an array of non-negative integers representing elevation heights, compute how much water can be trapped after rain.
Sliding Window Median
Given a list of integers and a positive window size k, return the median of each sliding window of length k.
Four Sum
Find all unique quadruplets in an array that sum to a given target.
Longest Repeating Character Replacement
Given a string and an integer k, return the length of the longest substring that can be made of identical characters after at most k replacements.
Count Nice Subarrays
Count subarrays that contain exactly k odd numbers.
Evaluate Reverse Polish Notation
Evaluate an arithmetic expression written in reverse Polish notation using a stack.
Daily Temperatures
Given a list of daily temperatures, return for each day how many days until a warmer temperature appears.
Decode String Stack
Decode a compressed string with repeated substrings like '3[a2[c]]' to 'accaccacc' using a stack-based approach.
Remove K Digits Stack
Remove k digits from a non-negative integer string to produce the smallest possible number using a stack-based approach.
Score of Parentheses
Given a balanced parentheses string, compute its score where () is 1 and concatenation sums, nesting multiplies by 2.
Validate Stack Sequences
Given two sequences of distinct integers, verify whether the second sequence is a valid pop order for a stack given the first sequence as push order.
Buildings with ocean view
Given building heights, return sorted indices of buildings that have a clear view of the ocean to their right.
Sum of Subarray Minimums
Given an integer array, sum the minimum of all contiguous subarrays modulo 10^9+7.
Showing 241–264 of 323 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.