将数组分成三个子数组的方案数

/*
 * Problem description:
 * We call a split of an integer array good if:
 * - The array is split into three non-empty continuous subarrays named left, mid, and right from left to right.
 * - The sum of elements in left is less than or equal to the sum of elements in mid, and the sum of elements in mid is less than or equal to the sum of elements in right.
 * Given a non-negative integer array nums, return the number of good ways to split nums. Since the answer may be large, return it modulo 10^9 + 7.
 */

import java.util.*;
import java.util.stream.*;

public class GoodSplitsStream {
    public int waysToSplit(int[] nums) {
        int MOD = 1_000_000_007;
        int n = nums.length;
        int[] prefixSum = new int[n + 1];
        for (int i = 0; i < n; i++) {
            prefixSum[i + 1] = prefixSum[i] + nums[i];
        }
        return IntStream.range(1, n).parallel().map(i -> {
            int leftSum = prefixSum[i];
            int left = binarySearch(prefixSum, i + 1, n, leftSum * 2);
            int right = binarySearch(prefixSum, i + 1, n, leftSum + (prefixSum[n] - leftSum) / 2 + 1);
            return right - left;
        }).reduce(0, (a, b) -> (a + b) % MOD);
    }

    private int binarySearch(int[] prefixSum, int low, int high, int target) {
        while (low < high) {
            int mid = (low + high) / 2;
            if (prefixSum[mid] < target) {
                low = mid + 1;
            } else {
                high = mid;
            }
        }
        return low;
    }
}