题解 | #【模板】栈#

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        Stack<Integer> stack = new Stack<>();
        for(int i = 0; i < n; i++){
            String str = in.next();
            if(str.equals("push")){
                int num = in.nextInt();
                stack.push(num);
            }else if(str.equals("pop")){
                if(stack.size() > 0){
                     System.out.println(stack.pop()); //弹出栈顶元素
                }else{
                    System.out.println("error");
                }
            }else if(str.equals("top")){
                 if(stack.size() > 0){
                     System.out.println(stack.peek()); //查看栈顶元素，不弹出
                }else{
                    System.out.println("error");
                }
            }
        }
    }
}