225.ImplementStackusingQueues.py

"""
    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).
"""
#Difficulty: Easy
#16 / 16 test cases passed.
#Runtime: 32 ms
#Memory Usage: 13.8 MB

#Runtime: 32 ms, faster than 57.12% of Python3 online submissions for Implement Stack using Queues.
#Memory Usage: 13.8 MB, less than 52.22% of Python3 online submissions for Implement Stack using Queues.

class MyStack:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.queue = []

    def push(self, x: int) -> None:
        """
        Push element x onto stack.
        """
        self.queue.append(x)
        length = len(self.queue) - 1
        while length:
            self.queue.append(self.queue.pop(0))
            length -= 1

    def pop(self) -> int:
        """
        Removes the element on top of the stack and returns that element.
        """
        if self.queue:
            return self.queue.pop(0)

    def top(self) -> int:
        """
        Get the top element.
        """
        if self.queue:
            return self.queue[0]

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