public static int shortPalindrome(string s)
{
    var (mod, occ1, occ2, occ3) = (1000000007, new int[26], new int[26, 26], new int[26, 26, 26]);
    return s.Select(c => c - 'a').Aggregate(0, (acc, c) =>
    {
        for (var i = 0; i < 26; i++)
        {
            acc = (acc + occ3[c, i, i]) % mod;
            occ3[i, c, c] = (occ3[i, c, c] + occ2[i, c]) % mod;
            occ2[i, c] = (occ2[i, c] + occ1[i]) % mod;
        }
        occ1[c]++;
        return acc;
    });
}