## Description

Implement the following operations of a stack using queues.

• push(x) – Push element x onto stack.
• pop() – Removes the element on top of the stack.
• top() – Get the top element.
• empty() – Return whether the stack is empty.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false


Notes:

• You must use only standard operations of a queue – which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

## Solutions

### 1. Queue

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

def __init__(self):
"""
"""
self.queue = []

def push(self, x: int) -> None:
"""
Push element x onto stack.
"""
self.queue.insert(0, x)
n = len(self.queue)
for _ in range(n-1):
self.queue.insert(0, self.queue.pop())

def pop(self) -> int:
"""
Removes the element on top of the stack and returns that element.
"""
return self.queue.pop()

def top(self) -> int:
"""
Get the top element.
"""
return self.queue[-1]

def empty(self) -> bool:
"""
Returns whether the stack is empty.
"""
return not any(self.queue)

# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.top()
# param_4 = obj.empty()
# 16/16 cases passed (32 ms)
# Your runtime beats 51.75 % of python3 submissions
# Your memory usage beats 100 % of python3 submissions (12.7 MB)