题解 | #从尾到头打印链表#

只需要三个变量 pre, cur,next，不需要额外空间，空间复杂度O(1)，代码如下：

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if (head == null || head.next == null){
            return head;
        }
        ListNode pre = head;
        ListNode cur = head.next;
        ListNode next = cur.next;
        pre.next = null; // 头节点指向null
        while(next != null){
            cur.next = pre;
            pre = cur;
            cur = next;
            next = next.next;
        }
        cur.next = pre;
        return cur;
    }
}