precourse - 1 harika

[harikakatne]

precourse - 1 harika

[super30admin]

I'll evaluate each exercise file separately since they appear to be different problems.

Exercise_1.py (Stack implementation using list):
Strengths:

• The basic structure of a stack is implemented with common operations: push, pop, peek, size, isEmpty, and show.
• The isEmpty method correctly checks if the stack is empty by checking the length.

Areas for improvement:

1. The isEmpty method should return a boolean (True/False) rather than returning the stack itself when empty. Currently, it returns the empty stack (which is truthy in Python) instead of False.
2. The show method is incorrect. It returns the first element immediately instead of showing all elements. It should return the entire stack or print it.
3. There's no time and space complexity comment at the top as requested.
4. The pop method could cause an IndexError if called on an empty stack. It should handle this case.
5. The peek method could cause an IndexError if called on an empty stack.
6. The init method should initialize the stack as an empty list, which is correct.

Time Complexity:

• push: O(1) amortized (due to list append)
• pop: O(1)
• peek: O(1)
• size: O(1)
• isEmpty: O(1)
• show: O(n) but currently implemented incorrectly

Space Complexity: O(n) where n is the number of elements in the stack.

Exercise_2.py (Stack implementation using linked list - but incorrect):
Strengths:

• The structure of a Node class is correct.
• The menu-driven interface for stack operations is provided.

Areas for improvement:

1. The Stack class incorrectly uses a list instead of a linked list. The problem likely expects a linked list implementation.
2. The push method uses "self.append(data)" which is not defined. It should add to the head of the linked list for O(1) time.
3. The pop method uses "self.pop()" which would cause infinite recursion. It should remove from the head.
4. There's no reference to the head node in the Stack class.
5. The Stack class doesn't maintain the linked list structure; it uses a list which defeats the purpose.
6. The pop method doesn't handle empty stack cases properly.

This implementation is fundamentally incorrect for a linked list-based stack.

Exercise_3.py (Singly linked list implementation):
Strengths:

• The basic structure of a singly linked list with append, find, and remove methods is attempted.
• The Node class (ListNode) is correctly defined.

Areas for improvement:

1. The append method has issues:
   • It creates a new node but doesn't correctly handle the case when the list is empty (the while loop will fail if head is None).
   • It should check if head is None and set head to the new node, then return.
   • The current code sets curr to self.head but then doesn't traverse correctly to the end.
2. The find method:
   • It never advances the curr pointer, so it will infinite loop if the head doesn't match the key.
   • It should advance curr in the loop: curr = curr.next.
3. The remove method:
   • It has several issues:
     • It compares self.head == key (should be self.head.data == key).
     • It doesn't handle the case when the key is not found.
     • The while loop condition is curr.next != None, which means the last node is never checked for key.
     • The pointer advancement inside the loop is not correct; it should advance only if no removal occurs.
4. The ListNode init method sets self.next to None, overriding the next parameter. It should be self.next = next.

Time Complexity:

• append: O(n) - correct but implementation is broken
• find: O(n) - but currently infinite loops
• remove: O(n) - but implementation is broken

Space Complexity: O(n) for n elements.

General feedback across all files:

• The code lacks comments and documentation. The problem asks to include Time and Space complexity at the top of each file.
• There are several syntax and logical errors that would prevent the code from running correctly.
• The student seems to have misunderstood the linked list implementation in Exercise_2 and Exercise_3.
• Edge cases (empty stack/list, operations on empty structures) are not handled properly.