Hard data structure and algorithm practice puzzle.
Given a list of [start, end] intervals, merge every pair that overlaps and return the result. Intervals that just touch, like [1, 4] and [4, 5], merge too. Sort by start time first. Then walk through once. If an interval starts before the last merged one ends, stretch that one's end. Otherwise, start a new one. The sort is what makes one pass enough.
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 Login Coverage Report, a pipeline you can run and download. See every Software Engineer project.
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