Remove Element

Filter into the front prefix

For stable removal, set write = 0 and scan each value. If it is not val, write it at nums[write] and increment write. The invariant is that the prefix before write contains exactly the non-target values seen so far in original order. Return write; the suffix is unspecified. This is O(n) time and O(1) extra space. If order is irrelevant and targets are common, use two ends: replace a target at the left with the last unchecked value and shrink the right boundary, but recheck the swapped value. Test no targets, all targets, targets at both ends, repeated values, and one-element input. Do not increment the left pointer after a swap without examining the replacement.