用两个栈实现一个队列

import java.util.Stack;

public class MyQueue {
    public Stack<Integer> s1;
    public Stack<Integer> s2;

    public MyQueue(Stack<Integer> s1, Stack<Integer> s2) {
        this.s1 = s1;
        this.s2 = s2;
    }

    public void add(int value){
        s1.push(value);
    }

    public int poll(){
        if(s1.isEmpty() && s2.isEmpty()){
            throw new RuntimeException("Queue is empty!");
        }else if(s2.isEmpty()){
            while(!s1.isEmpty()){
                s2.push(s1.pop());
            }
        }
        return s2.pop();
    }

    public int peek(){
        if(s1.isEmpty() && s2.isEmpty()){
            throw new RuntimeException("Queue is empty!");
        }else if(s2.isEmpty()){
            while(!s1.isEmpty()){
                s2.push(s1.pop());
            }
        }
        return s2.peek();
    }

    public static void main(String[] args) {
        MyQueue q = new MyQueue(new Stack<Integer>(), new Stack<Integer>());
        q.add(1);
        q.add(2);
        q.add(3);
        System.out.println(q.poll());
        System.out.println(q.poll());
        q.add(4);
        System.out.println(q.poll());
        System.out.println(q.peek());
    }
}