232.ImplementQueueusingStacks.py

"""
    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).
"""
#Difficulty: Easy
#17 / 17 test cases passed.
#Runtime: 44 ms
#Memory Usage: 13.7 MB

#Runtime: 44 ms, faster than 18.18% of Python3 online submissions for Implement Queue using Stacks.
#Memory Usage: 13.7 MB, less than 94.52% of Python3 online submissions for Implement Queue using Stacks.

class MyQueue:

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

    def push(self, x: int) -> None:
        """
        Push element x to the back of queue.
        """
        self.stack.append(x)
        self.stack.insert(0, self.stack.pop())

    def pop(self) -> int:
        """
        Removes the element from in front of queue and returns that element.
        """
        if self.stack:
            return self.stack.pop()

    def peek(self) -> int:
        """
        Get the front element.
        """
        if self.stack:
            return self.stack[-1]

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


# 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()