leetcode400

public class Solution {
    public int FindNthDigit(int n)
        {
            //StringBuilder sb = new StringBuilder();
            //for (int i = 1; i < int.MaxValue; i++)
            //{
            //    if (sb.Length < n)
            //    {
            //        sb.Append(i);
            //    }
            //    else
            //    {
            //        break;
            //    }
            //}

            //var k = sb.Length;

            //var s = sb.ToString().Substring(n - 1, 1);
            //var result = Convert.ToInt32(s);
            //return result;

            var result = 0;

            if (n >= 1 && n <= 9)//0~9
            {
                var dif = n - 0;
                var shang = dif;
                var yu = 0;

                var basic = (0 + shang).ToString();
                result = Convert.ToInt32(basic.Substring(yu, 1));
                //9*1
            }
            else if (n >= 10 && n <= 189)//10~99
            {
                var dif = n - 9;
                var shang = dif / 2;
                var yu = dif % 2;
                var next = 0;
                if (yu > 0)
                {
                    next = 1;
                }
                var basic = (9 + shang + next).ToString();
                if (yu > 0)
                {
                    result = Convert.ToInt32(basic.Substring(yu - 1, 1));
                }
                else
                {
                    result = Convert.ToInt32(basic.Substring(basic.Length - 1, 1));
                }
                //89*2
            }
            else if (n >= 190 && n <= 2889)//100~999
            {
                var dif = n - 189;
                var shang = dif / 3;
                var yu = dif % 3;
                var next = 0;
                if (yu > 0)
                {
                    next = 1;
                }
                var basic = (99 + shang + next).ToString();
                if (yu > 0)
                {
                    result = Convert.ToInt32(basic.Substring(yu - 1, 1));
                }
                else
                {
                    result = Convert.ToInt32(basic.Substring(basic.Length - 1, 1));
                }
                //899*3
            }
            else if (n >= 2900 && n <= 38889)//1000~9999
            {
                var dif = n - 2889;
                var shang = dif / 4;
                var yu = dif % 4;
                var next = 0;
                if (yu > 0)
                {
                    next = 1;
                }
                var basic = (999 + shang + next).ToString();
                if (yu > 0)
                {
                    result = Convert.ToInt32(basic.Substring(yu - 1, 1));
                }
                else
                {
                    result = Convert.ToInt32(basic.Substring(basic.Length - 1, 1));
                }
                //8999*4
            }
            else if (n >= 39000 && n <= 488889)//10000~99999
            {
                var dif = n - 38889;
                var shang = dif / 5;
                var yu = dif % 5;
                var next = 0;
                if (yu > 0)
                {
                    next = 1;
                }
                var basic = (9999 + shang + next).ToString();
                if (yu > 0)
                {
                    result = Convert.ToInt32(basic.Substring(yu - 1, 1));
                }
                else
                {
                    result = Convert.ToInt32(basic.Substring(basic.Length - 1, 1));
                }
                //89999*5
            }
            else if (n >= 490000 && n <= 5888889)//100000~999999
            {
                var dif = n - 488889;
                var shang = dif / 6;
                var yu = dif % 6;
                var next = 0;
                if (yu > 0)
                {
                    next = 1;
                }
                var basic = (99999 + shang + next).ToString();
                if (yu > 0)
                {
                    result = Convert.ToInt32(basic.Substring(yu - 1, 1));
                }
                else
                {
                    result = Convert.ToInt32(basic.Substring(basic.Length - 1, 1));
                }
                //899999*6
            }
            else if (n >= 5900000 && n <= 68888889)//1000000~9999999
            {
                var dif = n - 5888889;
                var shang = dif / 7;
                var yu = dif % 7;
                var next = 0;
                if (yu > 0)
                {
                    next = 1;
                }
                var basic = (999999 + shang + next).ToString();
                if (yu > 0)
                {
                    result = Convert.ToInt32(basic.Substring(yu - 1, 1));
                }
                else
                {
                    result = Convert.ToInt32(basic.Substring(basic.Length - 1, 1));
                }
                //8999999*7
            }
            else if (n >= 69000000 && n <= 788888889)//10000000~99999999
            {
                var dif = n - 68888889;
                var shang = dif / 8;
                var yu = dif % 8;
                var next = 0;
                if (yu > 0)
                {
                    next = 1;
                }
                var basic = (9999999 + shang + next).ToString();
                if (yu > 0)
                {
                    result = Convert.ToInt32(basic.Substring(yu - 1, 1));
                }
                else
                {
                    result = Convert.ToInt32(basic.Substring(basic.Length - 1, 1));
                }
                //89999999*8
            }
            else if (n >= 790000000 && n <= int.MaxValue)//100000000~999999999
            {
                var dif = n - 788888889;
                var shang = dif / 9;
                var yu = dif % 9;
                var next = 0;
                if (yu > 0)
                {
                    next = 1;
                }
                var basic = (99999999 + shang + next).ToString();
                if (yu > 0)
                {
                    result = Convert.ToInt32(basic.Substring(yu - 1, 1));
                }
                else
                {
                    result = Convert.ToInt32(basic.Substring(basic.Length - 1, 1));
                }
                //899999999*9
            }

            return result;
        }
}

https://leetcode.com/problems/nth-digit/#/description