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。如果两者均为空，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
    
      pop_stack;	stack
     
       push_stack;};