Remove Element / visualized

Two pointers travelling the same direction at different speeds: a slow write head marking the slot the next kept value goes into, and a fast read head scanning ahead. Anything that isn't val gets copied back to write; anything that is simply never gets copied. The invariant is the whole problem — nums[0, write) is a finished answer after every single step, the order of what survives is irrelevant, and everything past the returned length is junk.