反转链表

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
 public static ListNode ReverseList(ListNode head) {
            if(head==null)

            return null;

            ListNode reversedHead=null;
            ListNode current=head;
            ListNode tmp=null;
        
            //循环遍历节点
            while(current!=null){
                // 将节点赋予temp
            tmp= current;
            //
            current= current.next;
            //将节点尾指针置为空
            tmp.next=null;
            //判断反转链表是否为空
            if(reversedHead==null)
            reversedHead=tmp;
            //否则按照头插法插入
            else{
                tmp.next=reversedHead;
                reversedHead=tmp;
            }
            
     }
             //返回头节点
            return reversedHead;
     }
}