Medium data structure and algorithm practice puzzle.
Given two linked lists that join and share the same tail, passed in as their values, return the first value of that shared tail, or null if they don't share one. Start at the back of both and walk forward while the values match. The last match is where they join. Starting from the back means the different lengths don't matter.
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 Trade Undo Stack, a pipeline you can run and download. See every Software Engineer project.
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