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Pick a language, gear up your hero, and battle through short lessons you can run right on the page.
The friendliest first language — clean syntax, runs right here.
Boss
“Indentation is my domain. You will slip.”
Write and run your very first lines of Python, right here on the page.
5 min · Python
Store values in named boxes called variables, then reuse them.
6 min · Python
Use Python as a calculator — and do math with your variables.
6 min · Python
A 3-minute tour of one real problem: the statement, the test cases, and how your code is run and checked.
3 min
Real problems from the Lab. Solve 2 of these 3 to move on.
Join text together, measure it, and change its case.
7 min · Python
Real problems from the Lab. Solve 2 of these 3 to move on.
Hold many values under one name, and reach for the one you want.
7 min · Python
Real problems from the Lab. Solve 2 of these 3 to move on.
Run different code depending on whether something is true.
8 min · Python
Real problems from the Lab. Solve 2 of these 3 to move on.
Repeat work without writing it out a hundred times.
8 min · Python
Real problems from the Lab. Solve 2 of these 3 to move on.
Make your program read the numbers and text it is given, instead of typing them into the code.
7 min · Python
Package code under a name so you can reuse it.
9 min · Python
Real problems from the Lab. Solve 2 of these 3 to move on.
Store values under their own names instead of numbered positions.
8 min · Python
Real problems from the Lab. Solve 2 of these 3 to move on.
Python's shortcuts for transforming and filtering a list in one line.
9 min · Python
Real problems from the Lab. Solve 2 of these 3 to move on.
From here the Lab branches by topic: arrays, two pointers, graphs and more. Pick any branch.
Classic problem-solving patterns, ordered from easiest to hardest. Examples adapt to your language — every language track has a full translation. Finish a language track first, then work through these at your own pace. Work top to bottom, since each lesson gets a little harder. Skip until you're comfortable with variables, loops, and functions.
Boss
“Patterns are power. Prove you see them.”
Precompute running totals so any range sum becomes a quick subtraction.
6 min · Python examples
Use a dictionary to count how many times each value appears.
6 min · Python examples
Walk two indices through a sorted array to find pairs or ranges in one pass.
7 min · Python examples
Track a moving subarray window to answer range questions without restarting from scratch.
7 min · Python examples
Make the locally best choice at each step when that strategy is safe.
7 min · Python examples
Cut a sorted list in half each step to find a value in logarithmic time.
8 min · Python examples
Last-in-first-out structure for matching brackets, undo logic, and monotonic scans.
7 min · Python examples
Chain nodes with next pointers — traverse, count, and update without shifting a whole array.
8 min · Python examples
Explore a graph level by level — shortest steps in an unweighted grid or maze.
9 min · Python examples
Dive deep along one path before backtracking — great for exploring all reachable nodes.
9 min · Python examples
Represent nodes and edges with an adjacency list, then traverse to explore connections.
9 min · Python examples
Try choices, undo when they fail, and search all valid combinations.
10 min · Python examples
Break a problem into overlapping subproblems, remember answers, and reuse them.
10 min · Python examples