## DEV Community is a community of 700,827 amazing developers

We're a place where coders share, stay up-to-date and grow their careers.

# Implement Stack using a List.

CodeWithML
Sharing solutions to coding problems.
Originally published at codewithml.netlify.app ・2 min read

### Problem Statement: Implement stack with push, pop, peek, isEmpty, size methods using a List.

#### Test Cases

1. Push
• Push on empty stack.
• Push on non-empty stack
2. Pop
• Pop on empty stack --> None.
• Pop on non-empty stack. --> value
3. Peek
• Peek on empty stack --> None.
• Peek on non empty stack. --> value
4. isEmpty
• isEmpty on empty stack --> True
• isEmpty on non-empty stack --> False.
5. Size
• size on empty stack --> None.
• size on non empty stack. --> value

### Algorithm

1. Push
• Append new value to end of the list.
2. Pop
• If stack is empty,
• return None
• Else,
• Pop element from the end of list.
• return the element.
3. Peek
• If stack is empty,
• return None
• Else,
• return the last element of the list.
4. isEmpty
• If stack is empty,
• return True
• Else,
• return False
5. Size
• Return the length of the list.

### Time and Space complexity

1. Push
• Time complexity: O(1)
• Space complexity: O(1)
2. Pop
• Time complexity: O(1)
• Space complexity: O(1)
3. Peek
• Time complexity: O(1)
• Space complexity: O(1)
4. isEmpty
• Time complexity: O(1)
• Space complexity: O(1)
5. Size
• Time complexity: O(1)
• Space complexity: O(1)

### Code

``````class Stack(object):
def __init__(self):
self.items = []

def isEmpty(self):
return self.items == []

def push(self, data):
self.items.append(data)

def pop(self):
if self.isEmpty():
return None
return self.items.pop()

def peek(self):
if self.isEmpty():
return None
return self.items[-1]

def size(self):
return len(self.items)
``````

### Unit Test

``````import unittest
from stackLists import Stack

class TestStackLists(unittest.TestCase):
def test_end_to_end(self):
print('Test: Empty stack')
stack = Stack()
self.assertEqual(stack.peek(), None)
self.assertEqual(stack.pop(), None)

print('Test: One element')
stack = Stack()
stack.push(5)
self.assertEqual(stack.size(), 1)
self.assertEqual(stack.pop(), 5)
self.assertEqual(stack.peek(), None)

print('Test: More than one element')
stack = Stack()
stack.push(1)
stack.push(2)
stack.push(3)
self.assertEqual(stack.size(), 3)
self.assertEqual(stack.pop(), 3)
self.assertEqual(stack.peek(), 2)
self.assertEqual(stack.pop(), 2)
self.assertEqual(stack.peek(), 1)
self.assertEqual(stack.isEmpty(), False)
self.assertEqual(stack.pop(), 1)
self.assertEqual(stack.peek(), None)
self.assertEqual(stack.isEmpty(), True)
print('Success: test_end_to_end')

def main():
test = TestStackLists()
test.test_end_to_end()

if __name__ == '__main__':
main()
``````

Github repo