1.思路

用一个数组模拟栈，思路很简单，直接看代码就行

2.代码模板

import java.util.*;
public class Main {
    static final int N = 100010;    //模拟的栈大小
    static int[] s = new int[N];    //模拟的栈
    static Scanner in = new Scanner(System.in);
    static int tt = 0;  //指向当前栈顶元素
    static void push(int x) {
        s[++tt] = x;
    }
    static void pop() {
        tt--;
    }
    static boolean empty() {
        return tt == 0;
    }
    static int query() {
        return s[tt];
    }
    public static void main(String[] args) {
        String op = null;
        int m = in.nextInt();
        while (m --> 0) {
            op = in.next();
            switch (op) {
                case "push":
                    push(in.nextInt());
                    break;
                case "pop":
                    pop();
                    break;
                case "empty":
                    System.out.println(empty() ? "YES" : "NO");
                    break;
                case "query":
                    System.out.println(query());
                    break;
            }
        }
    }
}

3.复杂度分析

• 时间复杂度：O(1)
• 空间复杂度：O(n)