adding pre-course solutions

[nikhylw]

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[super30admin]

Exercise_1.py (List-based Stack)

• Strengths: Clean, simple implementation using Python's built-in list. All core operations (isEmpty, push, pop, peek, size) are correctly implemented. Good use of docstrings for time/space complexity.
• Issues: Minor - The show() method returns the internal list directly, which could allow external modification. Consider returning a copy for better encapsulation.
• Correctness: ✓ Correct
• Time Complexity: ✓ O(1) for all operations (amortized for push/pop)

Exercise_2.py (Linked List-based Stack)

• Strengths: Proper linked list implementation with Node class. Clean interactive menu interface. Good handling of empty stack case in pop.
• Issues: The pop operation returns None for empty stack but prints "Stack is empty." - this is handled correctly in the main loop, but the method itself doesn't distinguish between an empty stack and a valid None value being popped (if the stack allowed None values). Consider raising an exception instead for better error handling.
• Correctness: ✓ Correct
• Time Complexity: ✓ O(1) for both push and pop

Exercise_3.py (Singly Linked List)

• Strengths: Well-structured with clear docstrings. Proper handling of edge cases (empty list, head removal). Good use of helper method show() for visualization.
• Issues: The find() method returns the entire node object, which exposes internal implementation details. Consider returning just the data or a boolean indicating found/not found depending on requirements. The remove() method returns True/False but the caller doesn't check the return value in the example.
• Correctness: ✓ Correct
• Time Complexity: ✓ O(n) for all operations as documented

General Observations:

• All three exercises demonstrate solid understanding of fundamental data structures
• Code is well-commented with time/space complexity analysis
• Good separation of concerns (data structure class vs. usage example)
• Edge cases are generally handled appropriately
• Could benefit from adding type hints for better code documentation