Chapter 3 Stacks and Queues - 3.5

Problem 3.5: Implement a MyQueue class which implements a queue using two stacks.

The word "two stacks" is a strong hint. I implemented my solution quickly.

from stack import *
class MyQueue:
    def __init__(self):
        self.in_stack = stack()
        self.out_stack = stack()
    def enqueue(self, value):
        # If all elements are not in in_stack
        if self.in_stack.size() == 0:
            # Pop all elements from out_stack
            # and push them into in_stack
            while self.out_stack.size() != 0:
                self.in_stack.push(self.out_stack.pop())
        self.in_stack.push(value)
    def dequeue(self):
        # If all elements are not in out_stack
        if self.out_stack.size() == 0:
            # Pop all elements from in_stack
            # and push them into out_stack
            while self.in_stack.size() != 0:
                self.out_stack.push(self.in_stack.pop())
        return self.out_stack.pop()

# Test cases
if __name__ == "__main__":
    my_queue = MyQueue()
    for i in range(0, 5):
        print "Enqueue", i
        my_queue.enqueue(i)
    for i in range(0, 3):
        print "Dequeue", my_queue.dequeue()
    for i in range(10, 18):
        print "Enqueue", i
        my_queue.enqueue(i)
    for i in range(0, 11):
        print "Dequeue", my_queue.dequeue()

However, I never thought about optimizing it. Actually, there is a better and simpler solution on the answer page, which is quite similar to mine. (The ony difference is enqueue().)

from stack import *
class MyQueue:
    def __init__(self):
        self.in_stack = stack()
        self.out_stack = stack()
    def enqueue(self, value):
        self.in_stack.push(value)
    def dequeue(self):
        # If all elements are not in out_stack
        if self.out_stack.size() == 0:
            # Pop all elements from in_stack
            # and push them into out_stack
            while self.in_stack.size() != 0:
                self.out_stack.push(self.in_stack.pop())
        return self.out_stack.pop()

# Test cases
if __name__ == "__main__":
    my_queue = MyQueue()
    for i in range(0, 5):
        print "Enqueue", i
        my_queue.enqueue(i)
    for i in range(0, 3):
        print "Dequeue", my_queue.dequeue()
    for i in range(10, 18):
        print "Enqueue", i
        my_queue.enqueue(i)
    for i in range(0, 11):
        print "Dequeue", my_queue.dequeue()

The lesson: never stop thinking about optimization!!!