题解 | #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 {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param head ListNode类 
     * @return ListNode类
     */
    public ListNode ReverseList (ListNode head) {
        if (head == null) {
            return null;
        }
        // 定义俩个变量去翻转整个链表
        ListNode pre = null, current = head;
        while (current != null) {
            // 存的2,3
            ListNode nextNode = current.next;
            // 置空当前节点的下一节点为null,current = 1
            current.next = pre;
            pre = current;
            current = nextNode;
         }

        return pre;
    }
}