输入3，得到3+33+333=369的方法

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace jisuan
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("请输入2-9之间的数字:");
           // int num = Convert.ToInt32(Console.ReadLine());
            int sum = 0;
            string num1 = Console.ReadLine();
            //sum = StrJS(num1);
            sum = StrJsuan(num1);
            //sum = Jsuan(num);
            Console.WriteLine(sum);
            Console.ReadLine();

        }
        #region 方法一
        private static int  Jsuan(int num)
        {
            int temp = 0;
            int sum = 0;
            for (int i = 0; i < num; i++)
            {
                temp = temp * 10 + num;
                sum += temp;
            }
            return sum;
        }
        #endregion
        #region 方法二
        private static int StrJsuan(string num)
        {
            string aa = "";
            int sum = 0;
            for (int i = 0; i < Convert.ToInt32(num); i++)
            {
                aa += num;
                sum += Convert.ToInt32(aa);
            }
            return sum;
        }
        #endregion
        #region 方法三
        private static int StrJS(string num)
        {
            StringBuilder ss = new StringBuilder("");
            int sum = 0;
            for (int i = 0; i < Convert.ToInt32(num); i++)
            {
                ss.Append(num);
                sum += Convert.ToInt32(ss.ToString());
            }
            return sum;
        }
        #endregion
    }
}