Solution.cs

public class Solution {
    public int FindNthDigit(int n) {
        int k = 1, cnt = 9;
        while ((long) k * cnt < n) {
            n -= k * cnt;
            ++k;
            cnt *= 10;
        }
        int num = (int) Math.Pow(10, k - 1) + (n - 1) / k;
        int idx = (n - 1) % k;
        return num.ToString()[idx] - '0';
    }
}