反转链表

解法一、栈

import java.util.*;
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if (head == null)
            return null;
        Stack<ListNode> stack = new Stack<>();
        while (head != null) {
            stack.push(head);
            head = head.next;
        }
        ListNode nh = stack.peek();
        while (!stack.isEmpty()) {
            ListNode pre = stack.pop();
            if (!stack.isEmpty()) {
                pre.next = stack.peek();
            } else {
                pre.next = null;
            }

        }
        return nh;
    }
}