反转链表

思路：双指针+辅助指针，具体如下：定义help1和help2分别指向头节点和头节点的下一个节点，需要做的就是让help2指向下一个节点，辅助指针保存help2的信息，并指向help1，最后help1存贮temp的节点信息，循环条件就是help2不为null
代码

/*
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 help1 = head;
        ListNode help2 = head.next;
        while(help2!=null){
            ListNode temp = help2;
            help2 = help2.next;
            temp.next = help1;
            help1 = temp;
        }
        head.next = null;
        return help1;
    }
}