# Chapter 3 Stacks and Queues

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!!!;