# Question

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.

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false


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).

# Solution

class MyQueue(object):

def __init__(self):
"""
"""
self.instack = list()
self.outstack = list()

def push(self, x):
"""
Push element x to the back of queue.
:type x: int
:rtype: void
"""
self.instack.append(x)

def pop(self):
"""
Removes the element from in front of queue and returns that element.
:rtype: int
"""
if len(self.outstack) == 0:
self.__pour()
return self.outstack.pop()

def peek(self):
"""
Get the front element.
:rtype: int
"""
if len(self.outstack) == 0:
self.__pour()
return self.outstack[-1]

def empty(self):
"""
Returns whether the queue is empty.
:rtype: bool
"""
return len(self.instack) == 0 and len(self.outstack) == 0

def __pour(self):
while len(self.instack) > 0:
self.outstack.append(self.instack.pop())

# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()