【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）

代码实现

public class TwoStacksQueue {

    private Stack<Integer> stackPush;
    private Stack<Integer> stackPop;

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

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

    public int poll(){
        if (stackPop.isEmpty() && stackPush.isEmpty()) {
            System.out.println("queue is empty");
            return -1;
        }else if(stackPop.isEmpty()){
            while(!stackPush.isEmpty()){
                stackPop.push(stackPush.pop());
            }
        }
        return stackPop.pop();
    }

    public int peek(){
        if (stackPop.isEmpty() && stackPush.isEmpty()) {
            System.out.println("queue is empty");
            return -1;
        }else if(stackPop.isEmpty()){
            while(!stackPush.isEmpty()){
                stackPop.push(stackPush.pop());
            }
        }
        return stackPop.peek();
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        TwoStacksQueue queue = new TwoStacksQueue();
        int[] testNum = {4,2,4,6,5,0,1,10};
        for(int i:testNum){
            queue.add(i);
        }
        for(int i = 0; i < testNum.length; i++){
            System.out.println(queue.peek()+" "+queue.poll());
        }
    }

}