题解 | 【模板】栈

import java.util.*;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int num = in.nextInt();
        List<String> zn = new ArrayList<>();
        for (int i = 0; i < num; i++) {
            String result = in.next();
            if (result.equals("push")) {
                zn.add(in.next());
            }
            if (result.equals("pop")) {
                if (zn.size() != 0) {
                    System.out.println(zn.get(zn.size() - 1));
                    zn.remove(zn.size() - 1);
                } else {
                    System.out.println("error");
                }
            }
            if (result.equals("top")) {
                if (zn.size() != 0) {
                    System.out.println(zn.get(zn.size() - 1));
                } else {
                    System.out.println("error");
                }
            }
        }
    }
}