/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null) return head;

        ListNode pre = null;

        while (head!=null) {
            ListNode temp = head.next;
            head.next = pre;
            pre = head;
            head = temp;

        }
        return pre;

    }
}