Medium data structure and algorithm practice puzzle.
A dungeon generator has made n rooms and dug some corridors, each joining two rooms. Return the fewest extra corridors needed so every room can reach every other. That's the number of connected components minus one. Use union-find: each room starts as its own group, and each corridor merges two groups, unless both rooms are already in the same group, in which case the corridor just makes a loop.
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 Entity Stream Buffer, a pipeline you can run and download. See every Software Engineer project.
Browse all 576 puzzles