Hard data structure and algorithm practice puzzle.
Given a list of numbers and a target, return how many ways you can put a + or - in front of each number so they add up to the target. Recurse through the numbers, branching twice at each one: add it, or subtract it. At the end, count the paths that hit the target. That's 2^n paths in total.
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 Sampler, a pipeline you can run and download. See every Software Engineer project.
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