【题目】

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）。

package leif.algorithm_and_data_structure.stack_and_queue;

import java.util.Stack;

public class TwoStacksQueue {
    private Stack<Integer> stackPush = new Stack<Integer>();
    private Stack<Integer> stackPop = new Stack<Integer>();

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

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

        return stackPop.pop();
    }

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

        return stackPop.peek();
    }
}