Medium data structure and algorithm practice puzzle.
Given several sorted lists, merge them into one sorted list. Keep a pointer at the front of each list. Each round, find the smallest front value across all the lists, add it to the result, and move that list's pointer forward. Stop when every list is used up. A heap of the front values would make each round faster.
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 Track Parser, a pipeline you can run and download. See every Software Engineer project.
Browse all 576 puzzles