反转链表

在断裂之前将下一个节点保存下来

import java.util.Stack;
public class Solution {
public ListNode ReverseList(ListNode head) {
if(head == null ) return null;

ListNode pre = null;
ListNode next = null;

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

pre = head;
head = next;
}
return pre;
}
}