1573. Number of Ways to Split a String

public class Solution
{
    public int NumWays(string s)
    {
        const int kMod = 1_000_000_007;
        int ones = s.Count(c => c == '1');
        if (ones % 3 != 0)
            return 0;
        if (ones == 0)
        {
            long n = s.Length;
            return (int)((n - 1) * (n - 2) / 2 % kMod);
        }

        int s1End = -1;
        int s2Start = -1;
        int s2End = -1;
        int s3Start = -1;
        int onesSoFar = 0;

        for (int i = 0; i < s.Length; ++i)
        {
            if (s[i] == '1')
                ++onesSoFar;
            if (s1End == -1 && onesSoFar == ones / 3)
                s1End = i;
            else if (s2Start == -1 && onesSoFar == ones / 3 + 1)
                s2Start = i;
            if (s2End == -1 && onesSoFar == ones / 3 * 2)
                s2End = i;
            else if (s3Start == -1 && onesSoFar == ones / 3 * 2 + 1)
                s3Start = i;
        }

        return (int)((long)(s2Start - s1End) * (s3Start - s2End) % kMod);
    }
}