子数组的最大 GCD-Sum

/*
 * Leetcode Problem 2941: Maximum GCD-Sum of Subarray
 * 
 * Problem Description: 
 * Given an array of positive integers, find the maximum sum of any subarray such that the GCD (Greatest Common Divisor) of the elements in the subarray is greater than 1.
 * 
 * Solution Approach using Java Streams: 
 * 1. Generate all possible subarrays using IntStream.
 * 2. For each subarray, calculate the GCD using reduce method.
 * 3. Filter subarrays with GCD greater than 1 and calculate their sum.
 * 4. Track the maximum sum among all valid subarrays.
 */

import java.util.stream.IntStream;

public class MaximumGCDSubarraySumStream {
    // Helper method to compute the GCD of two numbers
    private static int gcd(int a, int b) {
        if (b == 0) {
            return a;
        }
        return gcd(b, a % b);
    }

    public static int maxGCDSum(int[] nums) {
        return IntStream.range(0, nums.length)
                .flatMap(i -> IntStream.rangeClosed(i, nums.length - 1)
                        .mapToObj(j -> IntStream.rangeClosed(i, j).map(k -> nums[k]).toArray())
                        .filter(subarray -> IntStream.of(subarray).reduce(nums[i], MaximumGCDSubarraySumStream::gcd) > 1)
                        .mapToInt(subarray -> IntStream.of(subarray).sum())
                )
                .max()
                .orElse(0);
    }

    public static void main(String[] args) {
        int[] nums = {3, 6, 9, 15};
        System.out.println(maxGCDSum(nums)); // Output: 33
    }
}