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.
只在push方法里倒騰一下數(shù)據(jù)单料， 原理相似于用隊(duì)列實(shí)現(xiàn)棧勺疼！
