Digit DP

/*dp[pos][cnt][flag] -> dp form with states*/

int solve(int b)
{
    num.clear();
    while (b > 0)
        num.pb(b % 10), b /= 10;
    reverse(all(num));
    return call(0, 0, 0);
}

int call(int pos, int cnt, int flag)
{
    if (cnt > k) return 0;
    
		if (pos == num.size())
    {
        if (cnt == k) return 1;
        else return 0;
    }

    if (f[pos][cnt][flag] != -1) return f[pos][cnt][flag];
    f[pos][cnt][flag] = 0;

    int limit = 9;
    if (flag == 0)
        limit = num[pos];

    for (int i = 0; i <= limit; i++)
    {
        int next_flag = flag, next_cnt = cnt;

        if (i < limit) next_flag = 1;
        if (i == d) next_cnt++;

        f[pos][cnt][flag] += call(pos + 1, next_cnt, next_flag);
    }
    return f[pos][cnt][flag];
}