add design-1 solutions

[nikhylw]

No description provided.

[super30admin]

Implement Hash Set (design_hashset.py)

Strengths:

• Good understanding of hash table concepts (chaining with linked lists)
• Clean separation of concerns with helper methods
• Proper handling of null storage buckets

Areas for Improvement:

1. Critical Bug: The getPrev method doesn't properly distinguish between "key found" and "key not found" cases. When the while loop exits because curr == None, you need to check if it was because the key was found (curr.key == key) or because we reached the end of the list.

2. Dummy Node Key Issue: Using -1 as a dummy key could conflict with valid keys (0 is valid per constraints). Consider using None or a separate isDummy flag.

3. Time Complexity: The solution has O(n) worst-case time complexity per operation, while the reference solution achieves O(1) average case using a two-level array approach. Consider using a larger number of buckets or a different collision resolution strategy.

4. Code Clarity: The getPrev method name is misleading since it returns a Node, not a boolean. Consider renaming to findNodeWithPrev or similar.

5. Edge Cases: The add operation doesn't check if the key already exists before adding, which could lead to duplicates.

VERDICT: NEEDS_IMPROVEMENT

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Implement Min Stack (design_min_stack.py)

Strengths:

• Excellent space optimization by using a single stack instead of two
• Correct handling of duplicate minimum values
• Clean, readable implementation
• Proper use of Python idioms

Areas for Improvement:

• Consider adding docstrings to explain the algorithm
• Could add type hints for better code documentation
• The condition val <= self.min is correct but could be clarified with a comment

Minor Note:
The reference solution uses a separate minSt stack which makes the logic more explicit, but the student's single-stack approach is equally valid and more memory-efficient. Both are accepted solutions for this problem.

VERDICT: PASS