232 Implement Queue using Stacks

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.

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).

The Idea: Maintain two stacks: one that will empty itself in reverse to the second stack, only when the second stack is empty.

Complexity: push()- O(1), pop() - amortized O(1), peek() - amortized O(1), empty() - O(1). We have constant amortized complexity because the amount of pushes will always be equal to the number pops, and the while loops ensure that future operations of popor peek will be available in O(1) time.

class MyQueue:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.s1, self.s2 = [], []

    def push(self, x):
        """
        Push element x to the back of queue.
        :type x: int
        :rtype: void
        """
        self.s1.append(x)

    def pop(self):
        """
        Removes the element from in front of queue and returns that element.
        :rtype: int
        """
        if len(self.s2) == 0:
            self._emptys1()
        return self.s2.pop()

    def peek(self):
        """
        Get the front element.
        :rtype: int
        """
        if len(self.s2) == 0:
            self._emptys1()
        return self.s2[-1]

    def empty(self):
        """
        Returns whether the queue is empty.
        :rtype: bool
        """
        return len(self.s1) == 0 and len(self.s2) == 0

    def _emptys1(self):
        while not len(self.s1) == 0:
            self.s2.append(self.s1.pop())


# 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()