Problem 5 SingleDigit, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            int n=0;
            n=Convert.ToInt32(Console.ReadLine());
            Sum obj = new Sum();
            int digitsum = obj.sumRecursive(n);
            Console.Write(digitsum);
        }
    
    }
    
    class Sum
    {
        
        public int sumRecursive(int num)
        {
            int sum =  sumDigits(num);
            if(sum < 10)
            {
                return sum;
            }
            sumRecursive(sum);
        }
        public int sumDigits(int num)
        { 
            
            if (num != 0)
            {
                return (num % 10 + sum(num / 10));
            }
            else
            {
                return 0;
            }
        }

}
}

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Compilation time: 0,12 s