Remove Duplicates from Sorted Array II

Compare against the second-last kept value

Keep write = 0 and scan every value. Copy the current value when write < 2 or when it differs from nums[write - 2]; then increment write. Because the input is sorted, equality with the value two positions behind means two copies are already retained, so a third would violate the limit. The invariant is that the written prefix is sorted and contains each processed value no more than twice. Runtime is O(n) and auxiliary space is O(1). Test empty input, one and two equal values, long duplicate runs, mixed runs, and values at the end. Generalize to at most k copies by copying when write < k or x != nums[write-k].