编写一个类，用两个栈实现队列，支持队列的基本操作（add、poll、peek）。

解题思路

为了实现栈后进先出的特点，设计栈stackPush用于实现add，栈stackPop用于实现poll。add操作永远在stackPush上执行；对于poll操作，如果stackPop有元素，则直接从stackPop弹出，否则先把元素从stackPush导入到stackPop，然后再从stackPop弹出。

实现代码

import java.util.Stack;

public class MyQueue {
    private Stack<Integer> stackPush;
    private Stack<Integer> stackPop;

    public MyQueue() {
        this.stackPush = new Stack<>();
        this.stackPop = new Stack<>();
    }

    public void add(int num) {
        stackPush.push(num);
    }

    public int poll() {
        if (stackPop.empty() && stackPush.empty()) {
            throw new RuntimeException("Queue is empty");
        }

        if (stackPop.empty()) {
            while (!stackPush.empty()) {
                stackPop.push(stackPush.pop());
            }
        }

        return stackPop.pop();
    }

    public int peek() {
        if (stackPop.empty() && stackPush.empty()) {
            throw new RuntimeException("Queue is empty");
        }

        if (stackPop.empty()) {
            while (!stackPush.empty()) {
                stackPop.push(stackPush.pop());
            }
        }

        return stackPop.peek();
    }
}