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) {
        // write code here
        Stack<ListNode> stack = new Stack<>();
        while(head != null){
            stack.push(head);
            head = head.next;
        }
        if(stack.isEmpty()){
            return null;
        }
        ListNode node = stack.pop();
        ListNode dummy = node;
        while(!stack.isEmpty()){
            ListNode temNode = stack.pop();
            node.next = temNode;
            node = node.next;
        }
        node.next = null;
        return dummy;
    }
}

1.使用栈先进后出的思想。Stack<ListNode> stack = new Stack<>()

2.然后把链表里的元素都存放在栈中

3.先保留头节点，一会直接打印

4.将node节点赋值并且穿起来，最后返回dummy即可