Hard data structure and algorithm practice puzzle.
Given n, return every balanced string of n pairs of parentheses. Build each string one character at a time with two rules. You can add "(" as long as you haven't used all n. You can add ")" only if there are more opens than closes so far. Those two rules never produce an invalid string.
Solve it three ways: snap the jigsaw pieces together, fill in the blanks, or type it from scratch against the tests. This one is free.
Every line of the solution comes with a note explaining why it is there, and a step by step replay shows the code running. At level 3, where you type it yourself, you can ask for an AI mock interview on this problem that ends with a hiring manager's scorecard.
Then grow it into a real project with the Software Engineer button: this puzzle becomes the Match Syntax Gate, a pipeline you can run and download. See every Software Engineer project.
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