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).

两个栈，一个作为压入栈，一个作为弹出栈。当弹出栈为空时，把压入栈中的数据依次弹出并压入到弹出栈中。

假设两者均为空，说明队列为空。

class Queue {

public:

    // Push element x to the back of queue.

    void push(int x) {

        push_stack.push(x);

    }

    // Removes the element from in front of queue.

    void pop(void) {

        if (pop_stack.empty())

		{

			while (!push_stack.empty())

			{

				pop_stack.push(push_stack.top());

				push_stack.pop();

			}

			if (!pop_stack.empty())

				pop_stack.pop();

		}

		else

		{

			pop_stack.pop();

		}

    }

    // Get the front element.

    int peek(void) {

        if (pop_stack.empty())

		{

			while (!push_stack.empty())

			{

				pop_stack.push(push_stack.top());

				push_stack.pop();

			}

			if (!pop_stack.empty())

				return pop_stack.top();

		}

		else

		{

			return pop_stack.top();

		}

		return 0;

    }

    // Return whether the queue is empty.

    bool empty(void) {

        if (pop_stack.empty() && push_stack.empty())

			return true;

		else

			return false;

    }

private:

	stack<int> pop_stack;

	stack<int> push_stack;

};