题解 | 由两个栈组成的队列

import java.util.*;

class MyStack1 {
    //设置两个栈
    private Stack<Integer> spush;
    private Stack<Integer> spop;

    public MyStack1(){
        this.spush=new Stack<Integer>();
        this.spop=new Stack<Integer>();
    }

    //只要pop栈为空，就可以直接往Push栈里增加数据
    public void add(int num){
        // 把pop栈的数据全部弹到push栈
        while (!spop.isEmpty()){
            spush.push(spop.pop());
        }

        spop.push(num);
    }

    //取最小值
    public int peek(){
        if(spop.isEmpty()&&spush.isEmpty()){
            throw new RuntimeException("all null");
        }
        //为了模拟队列先进先出，用push进，那么只能从Pop里弹
        //先把push的数据放入pop里，再从pop找最小值
        while (!spush.isEmpty()){
            spop.push(spush.pop());
        }
     return spop.peek();
    }

    //弹出pop的第一个数据
    public void poll(){
        peek();
        spop.pop();
    }

}
public class Main{
        public static void main(String[] args){
            Scanner scan = new Scanner(System.in);
            MyStack1 stack1 = new MyStack1();
            int t = scan.nextInt();
            for(int i=0;i<t;i++){
                String op=scan.next();
                if(op.equals("add")){
                    int x= scan.nextInt();
                    stack1.add(x);
                }else if(op.equals("peek")){
                    System.out.println(stack1.peek());
                }else if(op.equals("poll")){
                    stack1.poll();
                }
            }
        }
    }