https://mirror.codeforces.com/contest/1175/submission/238928196 For judging whether an array of length n is a 1~n permutation, I judged its sum{i^1} sum{i^2} sum{i^3} sum{i^4} and passed. Can anyone hack this solution or prove it correct? please help me :(
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It's incorrect.
Try this:
Maybe it will be difficult to construct a counterexample if you compute powers up to like $$$100$$$ and do some hashing.