implementQueueWithStacks.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).


class MyQueue:

    def __init__(self):
        """
        Initialize your data structure here.
        """

        self.s1 = []
        self.s2 = []

    def push(self, x: int) -> None:
        """
        Push element x to the back of queue.
        """
        self.s1.append(x)

    def pop(self) -> int:
        """
        Removes the element from in front of queue and returns that element.
        """
        self.peek()
        return self.s2.pop()

    def peek(self) -> int:
        """
        Get the front element.
        """
        if not self.s2:
            while self.s1:
                self.s2.append(self.s1.pop())
        return self.s2[-1]

    def empty(self) -> bool:
        """
        Returns whether the queue is empty.
        """
        return not self.s1 and not self.s2


# 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()