Implement Stack using Array, Implementing Stack using Linked List and Implement Singly Linked List

[anirudhv98]

Completed Precourse-1

[super30admin]

Exercise_1.java (Array-based Stack):

• Correctness: The implementation is mostly correct. The push method properly checks for overflow, and pop/peek handle underflow. However, there's an off-by-one error: top is initialized to 0, but in push, you're incrementing first then accessing a[top], which means you're writing to index 1 instead of 0. This causes the first element to be skipped.
• Time Complexity: O(1) for all operations - correct.
• Space Complexity: O(MAX) = O(n) where n is the maximum capacity - correct.
• Code Quality: Good structure with clear comments. The overflow/underflow messages are appropriate.

Exercise_2.java (Linked List-based Stack):

• Correctness: The approach of using a sentinel value (Integer.MIN_VALUE) to represent an empty stack is unconventional and could cause issues if Integer.MIN_VALUE is a valid data value. A better approach would be to use null to represent an empty stack.
• Time Complexity: O(1) for all operations - correct.
• Space Complexity: O(n) where n is the number of elements - correct.
• Code Quality: The sentinel value approach is a code smell. Using null would be more idiomatic and avoid potential data collision issues.

Exercise_3.java (Singly Linked List):

• Correctness: The implementation is correct. Insertion at the end and traversal work properly.
• Time Complexity: O(n) for insertion (traverses to end) and O(n) for printing - correct.
• Space Complexity: O(n) for the linked list storage - correct.
• Code Quality: Good structure. The code is readable and follows standard linked list patterns.

General Observations:

1. The student should add comments about time and space complexity at the top of each file as instructed.
2. The array-based stack has a critical bug that needs fixing.
3. The linked list stack should use null instead of a sentinel value for better design.