Medium data structure and algorithm practice puzzle.
Given n, return how many zeros are at the end of n factorial, without computing it. Every trailing zero comes from a 2 times a 5, and there are always more 2s, so just count the 5s. Add n / 5, plus n / 25, plus n / 125, and so on, rounding each down. The 25 term catches numbers like 25 that give two 5s.
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 Reading Stats Roll-up, a pipeline you can run and download. See every Software Engineer project.
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