两个栈实现队列

import java.util.Stack;

public class Problem_02_TwoStacksImplementQueue {

public static class TwoStacksQueue {
    public Stack<Integer> stackPush;
    public Stack<Integer> stackPop;

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

    // 核心函数  push栈向pop栈倒入数据  
    private void pushToPop() {
        if (stackPop.empty()) {
            while (!stackPush.empty()) {
                stackPop.push(stackPush.pop());
            }
        }
    }

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

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

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

public static void main(String[] args) {
    TwoStacksQueue test = new TwoStacksQueue();
    test.add(1);
    test.add(2);
    test.add(3);
    System.out.println(test.peek());
    System.out.println(test.poll());
    System.out.println(test.peek());
    System.out.println(test.poll());
    System.out.println(test.peek());
    System.out.println(test.poll());
}

}