题解 | #反转链表#

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
import java.util.*;
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if (head == null || head.next ==null){
            return head;
        }
        Stack<ListNode> stack  = new Stack<ListNode>();
        ListNode cur = head;
        while(cur!=null){
            stack.push(cur);
            System.out.println(cur.val);
            cur = cur.next;
        }
        head = stack.pop();
        cur = head;
        // System.out.println(head.val);
        while(!stack.isEmpty()){
            head.next = stack.pop();
            // System.out.println(head.next.val);
            head = head.next;
        }
	   // 最后的节点指向一定要为空，否则形成环
        head.next = null;
        return cur;
    }
}