206. Reverse Linked List.cs

// iteration
public class Solution {
    public ListNode ReverseList(ListNode head) {
        ListNode pre = null, temp = null;
        while (head != null) {
            temp = head.next;
            head.next = pre;
            pre = head;
            head = temp;
        }
        return pre;
    }
}

// recursion
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode newHead = ReverseList(head.next);
        head.next.next = head;
        head.next = null;
        return newHead;
    }
}