# 232 Implement Queue using Stacks

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).
The Idea: Maintain two stacks: one that will empty itself in reverse to the second stack, only when the second stack is empty.
Complexity: `push()`- O(1), `pop()` - amortized O(1), `peek()` - amortized O(1), `empty()` - O(1). We have constant amortized complexity because the amount of pushes will always be equal to the number pops, and the while loops ensure that future operations of `pop`or `peek` will be available in O(1) time.
class MyQueue:
def __init__(self):
"""
"""
self.s1, self.s2 = [], []
def push(self, x):
"""
Push element x to the back of queue.
:type x: int
:rtype: void
"""
self.s1.append(x)
def pop(self):
"""
Removes the element from in front of queue and returns that element.
:rtype: int
"""
if len(self.s2) == 0:
self._emptys1()
return self.s2.pop()
def peek(self):
"""
Get the front element.
:rtype: int
"""
if len(self.s2) == 0:
self._emptys1()
return self.s2[-1]
def empty(self):
"""
Returns whether the queue is empty.
:rtype: bool
"""
return len(self.s1) == 0 and len(self.s2) == 0
def _emptys1(self):
while not len(self.s1) == 0:
self.s2.append(self.s1.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()