## Description

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).

## Solutions

### 1. Two Stacks

# Time: O(n)
# Space: O(n)
class MyQueue:

def __init__(self):
"""
"""
self.push_stack = []
self.helper = []

def pour(self, push_stack, helper):
if not any(push_stack):
return
while push_stack:
helper.append(push_stack.pop())

def push(self, x: int) -> None:
"""
Push element x to the back of queue.
"""
self.push_stack.append(x)

def pop(self) -> int:
"""
Removes the element from in front of queue and returns that element.
"""
if not any(self.push_stack) and not self.helper:
return None

if not any(self.helper):
self.pour(self.push_stack, self.helper)

return self.helper.pop()

def peek(self) -> int:
"""
Get the front element.
"""
if not any(self.push_stack) and not self.helper:
return None

if not any(self.helper):
self.pour(self.push_stack, self.helper)

return self.helper[-1]

def empty(self) -> bool:
"""
Returns whether the queue is empty.
"""
if not any(self.push_stack) and not self.helper:
return True
else:
return False

# 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()
# 17/17 cases passed (20 ms)
# Your runtime beats 98.55 % of python3 submissions
# Your memory usage beats 100 % of python3 submissions (12.8 MB)