jz9.翻转链表

输入一个链表，反转链表后，输出新链表的表头。

/*
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 = null;
        ListNode next = null;
        //这个是head！=null
        while(head!=null){
            //记录下一次要翻转的结点
            next = head.next;
            //
            head.next = pre;
            pre = head;
            head = next;
        }
        return pre;
        

    }
}