#yyds干货盘点# LeetCode程序员面试金典：化栈为队

题目：

实现一个MyQueue类，该类用两个栈来实现一个队列。

示例：

MyQueue queue = new MyQueue();

queue.push(1);

queue.push(2);

queue.peek();  // 返回 1

queue.pop();   // 返回 1

queue.empty(); // 返回 false

代码实现：

class MyQueue {
    Deque<Integer> inStack;
    Deque<Integer> outStack;

    public MyQueue() {
        inStack = new ArrayDeque<Integer>();
        outStack = new ArrayDeque<Integer>();
    }

    public void push(int x) {
        inStack.push(x);
    }

    public int pop() {
        if (outStack.isEmpty()) {
            in2out();
        }
        return outStack.pop();
    }

    public int peek() {
        if (outStack.isEmpty()) {
            in2out();
        }
        return outStack.peek();
    }

    public boolean empty() {
        return inStack.isEmpty() && outStack.isEmpty();
    }

    private void in2out() {
        while (!inStack.isEmpty()) {
            outStack.push(inStack.pop());
        }
    }
}