gold-3-4

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

说明：

• 你只能使用标准的栈操作 – 也就是只有 push to top, peek/pop from top, size 和 is empty 操作是合法的。
• 你所使用的语言也许不支持栈。你可以使用 list 或者 deque（双端队列）来模拟一个栈，只要是标准的栈操作即可。
• 假设所有操作都是有效的 （例如，一个空的队列不会调用 pop 或者 peek 操作）。

//leetcode submit region begin(Prohibit modification and deletion)
class MyQueue {

    private Stack<Integer> inStack;
    private Stack<Integer> outStack;

    /** Initialize your data structure here. */
    public MyQueue() {
        this.inStack = new Stack<>();
        this.outStack = new Stack<>();
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        this.inStack.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if (outStack.isEmpty()) {
            transferStack();
        }

        if (outStack.isEmpty()) {
            return -1;
        } else {
            return outStack.pop();
        }
    }

    /** Get the front element. */
    public int peek() {
        if (outStack.isEmpty()) {
            transferStack();
        }

        if (outStack.isEmpty()) {
            return -1;
        } else {
            return outStack.peek();
        }
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return this.inStack.isEmpty() && this.outStack.isEmpty();
    }

    private void transferStack() {
        while (!inStack.isEmpty()) {
            int data = inStack.pop();
            outStack.push(data);
        }
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */
//leetcode submit region end(Prohibit modification and deletion)