Recursive Reversal

The recursive approach reverses the rest of the list first, then fixes the current node:

function reverse(head):
    if head == null or head.next == null:
        return head
    newHead = reverse(head.next)
    head.next.next = head
    head.next = null
    return newHead

The insight: after reverse(head.next) returns, head.next still points to what was the second node. That node is now the tail of the reversed suffix. So head.next.next = head appends head to the end.

Time: $O(n)$. Space: $O(n)$ for the call stack.

The iterative version is more space-efficient, but the recursive version is cleaner to write.