Find All Numbers Disappeared in an Array / visualized

The values are in 1..n and the array has length n, so the array is almost a permutation of its own index set — and that is a data structure, not a coincidence. Value v has exactly one correct home: index v−1. Pass one walks i and repeatedly swaps whatever is sitting at i into its home slot until nothing more can move. Pass two just reads off every index that ended up not holding index+1; those are the missing numbers. This page shows the swap form because it is the one that generalises — the same loop, plus a range check, solves LeetCode 41. The other accepted answer is negative marking: walk the array and flip nums[abs(v)-1] negative to record "v was seen", then report every still-positive index. It is shorter and never moves an element, but it only works because these values are guaranteed positive and in range, and it destroys the sign bit. Cyclic sort survives negatives, zeros and huge values; negative marking does not.