Easy data structure and algorithm practice puzzle.
Given a string, return the index of the first character that appears only once, or -1 if there isn't one. Make two passes. First, count every character. Then walk the string again from the left and return the first index whose count is exactly 1. The count has to come first, because you can't know a character is unique until you've seen the whole string.
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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