Posted on Oct 4, 2020 • Originally published at codewithml.netlify.app

Implement Stack using a List.

#challenge #python

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

Test Cases

Push
- Push on empty stack.
- Push on non-empty stack
Pop
- Pop on empty stack --> None.
- Pop on non-empty stack. --> value
Peek
- Peek on empty stack --> None.
- Peek on non empty stack. --> value
isEmpty
- isEmpty on empty stack --> True
- isEmpty on non-empty stack --> False.
Size
- size on empty stack --> None.
- size on non empty stack. --> value

Algorithm

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

Time and Space complexity

Push
- Time complexity: O(1)
- Space complexity: O(1)
Pop
- Time complexity: O(1)
- Space complexity: O(1)
Peek
- Time complexity: O(1)
- Space complexity: O(1)
isEmpty
- Time complexity: O(1)
- Space complexity: O(1)
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()