Remove Duplicates from Sorted Array / visualized

The same slow write / fast read machine as LC 27, with one extra idea: what you compare against. Sorting puts equal values side by side, so a value is worth keeping exactly when it differs from the last value you actually kept — nums[write − 1], not nums[read-1]. The invariant carries the proof: nums[0, write) is sorted and duplicate-free after every step, so the comparison only ever needs one look back.