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