232. Implement queue with stack

232. Using stack to realize queue
     Please use only two stacks to implement the FIFO queue . The queue should support all operations supported by the general queue （push、pop、peek、empty）：

Push ： Simulate queuing
Out of the stack ： Simulate the queue
push： Append an element after the queue
pop： Delete the first element of the queue
peek： Get the first element of the queue

Be careful ：
1、 Initialize two stacks , Must be outside the method , It's a class variable .
2、 Outgoing queue , When you move elements from in stack to out stack , You must ensure that there are no elements in the stack , otherwise , When adding an element again , It will cause disorder .