题解 | 【模板】栈的操作

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Deque<Integer> stack = new ArrayDeque<>();
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        while(n-->0){
           String s = in.next();
           if(s.equals("push")){
            int x = in.nextInt();
            stack.push(x);
            continue;
           }
           if(s.equals("size")){
            System.out.println(stack.size());
            continue;
           }
           if(s.equals("pop")&&!stack.isEmpty()){
            stack.pop();
            continue;
           }
           if(s.equals("pop")&&stack.isEmpty()){
            System.out.println("Empty");
            continue;
           }
           if(s.equals("query")&&!stack.isEmpty()){
            System.out.println(stack.peek());
            continue;
           }
           if(s.equals("query")&&stack.isEmpty()){
            System.out.println("Empty");
            continue;
           }
        }
    }
}