Zero-Based Permutation Validation

Question

nums is an integer array holding n elements. It counts as a permutation of [0, 1, 2, ..., n-1] when each integer from 0 up to n-1 shows up in it exactly once, in any order. Some entries can lie outside that range. Rearranging or overwriting nums is fine, but apart from the input the solution may only use O(1) extra space. ...