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 || head.next == null || head.next.next == null) {
            return head;
        }
        ListNode preNode = head;
        ListNode current = head.next;
        ListNode nextNode = null;
        //最开始的头节点指向空
        preNode.next = null;

        while (current != null) {
            //先获取下一个节点
            nextNode = current.next;
            //当前节点指向上一个节点
            current.next = preNode;
            //上一个节点赋值为当前节点
            preNode = current;
            //当前节点往后走一步
            current = nextNode;
        }

        return preNode;
    }
}