Remove Duplicates from Sorted Array II, Merge Sorted Array and Search a 2D Matrix II

[anirudhv98]

Completed Two-Pointers-2, please review.

[super30admin]

Merging of 2 arrays (Problem1.cs)

Looking at this evaluation, I need to assess the student's solution against the problem requirements.

Problem Understanding:
The problem asks to merge two sorted arrays nums1 and nums2 into a single sorted array stored in nums1. The first m elements of nums1 contain valid data, and the last n elements are placeholders (0s). The solution should run in O(m+n) time.

Student's Solution Analysis:
The student has submitted a solution for "Remove Duplicates from Sorted Array II" (LeetCode 80), which is a completely different problem. Their code:

• Uses two pointers (slow and fast)
• Counts occurrences of duplicates
• Allows at most 2 occurrences of each element
• Returns the length of the resulting array

This solution does NOT address the "Merge Sorted Array" problem at all. It solves a different problem about removing duplicates while allowing at most 2 duplicates per element.

Evaluation:

1. Correctness: The solution is incorrect for the given problem. It solves a different problem entirely.

2. Time Complexity: O(n) - but this is for the wrong problem.

3. Space Complexity: O(1) - good, but again, for the wrong problem.

4. Code Quality: The code is well-structured with clear comments and good variable naming. The swap method is clean.

5. Efficiency: The algorithm is efficient for the problem it solves, but not applicable here.

Final Verdict: NEEDS_IMPROVEMENT

The student has solved the wrong problem. They need to implement the merge algorithm using three pointers: one for the end of the valid portion of nums1, one for the end of nums2, and one for the current position to fill in nums1 (starting from the end). The algorithm should compare elements from both arrays and place the larger one at the current position, working backwards to avoid overwriting valid data.

VERDICT: NEEDS_IMPROVEMENT

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Search 2D sorted matrix II (Problem2.cs)

• Major Issue: You submitted code for "Merge Sorted Array" (LeetCode 88) instead of "Search 2D Matrix II" (LeetCode 240). These are completely different problems.
• The solution you provided would need to be completely replaced with an algorithm that searches a 2D matrix where rows and columns are sorted.
• For the correct problem, consider starting from the top-right corner or bottom-left corner and eliminating rows/columns based on comparisons with the target.
• Alternatively, you could perform binary search on each row (as shown in the reference solution with O(m log n) complexity).

VERDICT: NEEDS_IMPROVEMENT

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Edit and Remove Duplicates in an array (Problem3.cs)

• Major Issue: You solved a completely different problem. The problem you were given asks to remove duplicates in-place from a sorted array so that each element appears at most twice. Your solution searches for a target value in a matrix.
• Correctness: Your solution is correct for Search Matrix but completely fails to address the Edit and Remove Duplicates problem.
• Time Complexity: Your solution has O(m+n) complexity for the matrix problem, but this is irrelevant since it's the wrong problem.
• Space Complexity: O(1) is correct for your implemented solution, but again, wrong problem.
• Code Quality: Your code is well-written with clear comments and proper structure for the matrix search problem.

You need to implement a solution that:

1. Uses a two-pointer approach (slow and fast pointers)
2. Keeps track of element frequency
3. Allows at most 2 occurrences of each element
4. Modifies the array in-place with O(1) extra space

Study the reference solution to understand how to track counts and overwrite elements in-place.

VERDICT: NEEDS_IMPROVEMENT