Easy data structure and algorithm practice puzzle.
Given an array of numbers and a target, return the indices of the two numbers that add up to the target. There's always exactly one answer. Checking every pair works, but it's O(n squared). Instead, walk the array once and store each number you've seen in a hash map. For each new number, check whether target minus it is already there. One lookup per number makes it O(n).
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 Trip Intake Pipeline, a pipeline you can run and download. See every Software Engineer project.
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