Medium data structure and algorithm practice puzzle.
Given n, return the nth Fibonacci number, where fib(0) is 0, fib(1) is 1, and every number after is the sum of the two before it. Plain recursion recomputes the same values over and over and takes exponential time. Keep just the last two numbers and loop forward instead. That's 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 Read Calibrator, a pipeline you can run and download. See every Software Engineer project.
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