from research online

disc intersections

class Solution {
    public int solution(int[] a) {
    int result = 0;
    int[] dps = new int[a.length];
    int[] dpe = new int[a.length];

    for (int i = 0; i < a.length; i++)
    {
        dps[Math.max(0, i - a[i])]++;
        dpe[Math.min(a.length - 1, i + a[i])]++;
    }

    int t = 0;
    for (int i = 0; i < a.length; i++)
    {
        if (dps[i] > 0)
        {
            result += t * dps[i];
            result += dps[i] * (dps[i] - 1) / 2;
            t += dps[i];
        }
        t -= dpe[i];
    }
    if(result>10000000) return -1;

    return result;

    }
}