Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack -- which means only
  push to top, peek/pop from top, size, and
is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

题目要求通过栈来模拟队列的行为。与此类似的还有通过队列模拟栈（http://blog.csdn.net/sunao2002002/article/details/46482673），此题是算法导论第十章的一道题。算法例如以下：

堆栈a和b。a用作入队，b出队

（1）判队满：假设a满且b不为空，则队满

（2）判队空：假设a和b都为空，则队空

（3）入队：首先判队满。

若队不满：（1）栈a若不满，则直接压入栈a

(2)若a满，则将a中的全部元素弹出到栈b中，然后再将元素入栈a

（4）出队：(1)若b空就将a中的全部元素弹出到栈b中。然后出栈

（2）b不空就直接从b中弹出元素

代码例如以下：

class Queue {

public:

	// Push element x to the back of queue.

	stack<int> in;

	stack<int> out;

	void push(int x) {

		in.push(x);

	}

	void move(){

		while(!in.empty())

		{

			int x = in.top();

			in.pop();

			out.push(x);

		}

	}

	// Removes the element from in front of queue.

	void pop(void) {

		if (out.empty())

		{

			move();

		}

		if (!out.empty())

		{

			out.pop();

		}

	}

	// Get the front element.

	int peek(void) {

		if (out.empty())

		{

			move();

		}

		if (!out.empty())

		{

			return out.top();

		}

	}

	// Return whether the queue is empty.

	bool empty(void) {

		return in.empty() && out.empty();

	}

};