Medium data structure and algorithm practice puzzle.
Given an array, return for each value the next bigger value to its right, or -1 if there isn't one. Use a stack of indices still waiting for an answer. For each new value, pop every waiting index with a smaller value and give it this value as its answer. Then push the new index. Each one goes on and off the stack once.
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 Clause Undo Stack, a pipeline you can run and download. See every Software Engineer project.
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