Fast & Slow Pointers

One skeleton, three questions: is there a cycle, where does it start, what's the middle.

Move fast two steps per iteration and slow one step; when fast reaches the end, slow lands on the middle (second middle when length is even).

Repeatedly map n to the sum of squares of its digits; a happy number eventually reaches 1; otherwise the sequence cycles — detect the cycle with a hash set…

For O(1) extra space, find the middle with fast/slow, reverse the second half, then compare the first half with the reversed half node by node.

Because the list is sorted, duplicates appear next to each other — skip next when current.val == current.next.val without a hash set.