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 = ;

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

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

            return result;
        }
}

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