solution.js

/**
 * @param {number[]} nums
 * @return {number}
 */
var waysToSplit = function (nums) {
    const prefixSum = new Array(nums.length + 1);
    prefixSum[0] = 0;
    let result = 0;
    const mod = 10 ** 9 + 7;
    for (let i = 0; i < nums.length; i++) {
        prefixSum[i + 1] = prefixSum[i] + nums[i];
    }
    for (let i = 0; i < nums.length - 2; i++) {
        const leftSum = prefixSum[i + 1];

        const midLeft = find1(prefixSum, leftSum, i + 1, nums.length - 2);

        if (midLeft === -1) {
            break;
        }
        const midRight = find2(prefixSum, i + 1, midLeft, nums.length - 2);
        if (midRight === -1) {
            continue;
        }

        result = (result + midRight - midLeft + 1) % mod;
    }
    return result;
};

function find1 (prefixSum, leftSum, start, high) {
    let low = start;
    let result = Infinity;
    while (low <= high) {
        const mid = (low + high) >> 1;
        const midSum = prefixSum[mid + 1] - prefixSum[start];
        if (midSum >= leftSum) {
            result = Math.min(result, mid);
            high = mid - 1;
        } else {
            low = mid + 1;
        }
    }
    if (result === Infinity) {
        return -1;
    }
    return result;
}

function find2 (prefixSum, start, low, high) {
    let result = -1;
    while (low <= high) {
        const mid = (low + high) >> 1;
        const rightSum = prefixSum[prefixSum.length - 1] - prefixSum[mid + 1];
        const midSum = prefixSum[mid + 1] - prefixSum[start];
        if (rightSum < midSum) {
            high = mid - 1;
        } else {
            result = Math.max(result, mid);
            low = mid + 1;
        }
    }

    return result;
}