Given an array nums of length n where every value is between 1 and n, and each value appears either once or twice, return every value that appears twice. The array's own values are secretly valid indices into itself — that's the constraint a brute-force scan throws away. Treat each value as a pointer to a slot, and flip the sign of what's stored there the first time you visit it. A negative sign IS the "already seen" flag, so a second visit to the same slot is instantly recognizable — no hash setHash SetA collection that lets you check "have I seen this value before?" in O(1) time, normally backed by extra memory. This problem's constraints let the input array itself play that role. required, and the whole scan finishes in a single O(n) pass.