The Self-Taught Computer Scientist

Data Structures
118
Otherwise, you remove the first item in your linked list and return it:
poppednode = self.head
self.head = self.head.next
return poppednode.data
Here is an example of creating a stack using this code and pushing and popping items on and 
off of it:
stack = Stack()
stack.push(1)
stack.push(2)
stack.push(3)
for i in range(3):
print(stack.pop())
>> 3
>> 2
>> 1
Finally, you can also use a Python list as a stack. Here is how it works:
stack = []
print(stack)
stack.append('Kanye West')
print(stack)
stack.append('Jay- Z')
print(stack)
stack.append('Chance the Rapper')
print(stack)
stack.pop()
print(stack)
>> []
>> ['Kanye West']
>> ['Kanye West', 'Jay- Z']
>> ['Kanye West', 'Jay- Z', 'Chance the Rapper']
>> ['Kanye West', 'Jay- Z']
Python’s lists come with the methods 
append
and 
pop
. The 
append
method adds an item to the end 
of a list, which is the same as pushing an item onto a stack. The 
pop
method removes an item from 
the end of a list. If you don’t specify which item to remove, it removes the last item.
When you first print your stack, it is empty because you haven’t added anything to it yet.
>> []

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Chapter 11 Stacks
119
Next, you push three items— 
'Kanye West'
, 
'Jay- Z'
, and 
'Chance the Rapper'
— onto your stack 
with these lines of code:
stack.append('Kanye West')
stack.append('Jay- Z')
stack.append('Chance the Rapper')
Then, you pop the last element, 
'Chance the Rapper'
, off your stack, leaving the first two elements:
stack.pop()
That is why when you print your stack for the final time, 
'Chance the Rapper'
is missing:
>> ['Kanye West', 'Jay- Z']
Of course, when you use a Python list as a stack, you are not limited to taking items off your stack 
in the order you put them on, so if you want to enforce that, you need to create a 
Stack
class.
Using Stacks to Reverse Strings
A common interview question for Python jobs is to reverse a string three different ways. If you are 
familiar with Python, you know you can reverse a string like this:
a_string[::- 1]
or like this:
''.join(reversed('a string'))
When it comes to reversing a string a third way, you may be stuck, however. The key is what you 
learned earlier in this chapter: you can use a stack to reverse a string because when you pop characters 
off a stack, they come off the stack in reverse order (see Figure 11.3).