Remove Nth Node From End of List

Write `remove_nth_from_end(head, n)` removing the nth node counted from the end, and returning the head. [1,2,3,4,5], n = 2 -> [1,2,3,5] Counting the length first and walking again is two passes. Instead open a gap of n nodes between two pointers and advance both until the leader falls off — the trailing pointer is then exactly where you need it. Start from a dummy head so that removing the first node needs no separate branch.