Easy data structure and algorithm practice puzzle.
Given a string of brackets ( ) [ ] { }, return true if they all close in the right order. Use a stack. Push every opening bracket. When you hit a closing bracket, the top of the stack must be its match, so pop it. At the end, the stack has to be empty. Every compiler and JSON parser does this check.
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 Document Undo Stack, a pipeline you can run and download. See every Software Engineer project.
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