Implement Maximum Sum of Distinct Subarrays With Length K (LeetCode 2461)

[Copilot]

This PR adds a complete solution for the "Maximum Sum of Distinct Subarrays With Length K" problem, which finds the maximum subarray sum among all subarrays of length k where all elements are distinct.

Problem Description

Given an integer array nums and an integer k, find the maximum subarray sum of all subarrays that meet these conditions:

• The subarray has exactly k elements
• All elements in the subarray are distinct

Return the maximum sum, or 0 if no valid subarray exists.

Examples

// Example 1
nums := []int{1, 5, 4, 2, 9, 9, 9}
k := 3
// Valid subarrays: [1,5,4] (sum=10), [5,4,2] (sum=11), [4,2,9] (sum=15)
// Output: 15

// Example 2  
nums := []int{4, 4, 4}
k := 3
// No valid subarrays since all contain duplicates
// Output: 0

Solution

The implementation uses an efficient sliding window approach with O(n) time complexity:

1. Sliding Window: Maintains a window of exactly k elements
2. Frequency Tracking: Uses a hash map to count element occurrences in the current window
3. Distinct Check: A window is valid only when the frequency map size equals k (all elements distinct)
4. Sum Optimization: Tracks the maximum sum among all valid windows

Key Features

• Efficient: O(n) time complexity, O(k) space complexity
• Robust: Handles all edge cases including negative numbers, k > array length, and arrays with no valid subarrays
• Well-tested: Comprehensive test suite with 10 test cases covering various scenarios
• Standards Compliant: Follows the repository's established patterns and Go coding conventions

Files Added

• solutions/2461_maximum_subarray_sum_distinct_elements/maximum_subarray_sum.go - Core implementation
• solutions/2461_maximum_subarray_sum_distinct_elements/maximum_subarray_sum_test.go - Comprehensive test suite

All existing tests continue to pass, ensuring no regressions were introduced.

This pull request was created as a result of the following prompt from Copilot chat.

Solve the following LeetCode problem:

Problem: Find the maximum subarray sum of all subarrays of length k where all elements are distinct.

Description:
You are given an integer array nums and an integer k. Find the maximum subarray sum of all the subarrays of nums that meet the following conditions:

• The length of the subarray is k, and
• All the elements of the subarray are distinct.

Return the maximum subarray sum of all the subarrays that meet the conditions. If no subarray meets the conditions, return 0.

Examples:

1. Input: nums = [1,5,4,2,9,9,9], k = 3
   Output: 15
   Explanation: Valid subarrays are [1,5,4] (sum=10), [5,4,2] (sum=11), [4,2,9] (sum=15)

2. Input: nums = [4,4,4], k = 3
   Output: 0
   Explanation: No valid subarrays since all contain duplicate elements

Algorithm Approach:
Use sliding window technique with a hash map to track element frequencies:

1. Maintain a window of size k
2. Use a map to count occurrences of each element in the current window
3. Track the number of distinct elements in the window
4. If all k elements are distinct, calculate sum and update maximum
5. Slide the window by removing the leftmost element and adding the next element

Implementation Requirements:

• Function signature: func maximumSubarraySum(nums []int, k int) int64
• Handle edge cases where no valid subarray exists
• Use efficient sliding window approach with O(n) time complexity
• Include comprehensive test cases covering all examples

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