count the string, 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)
        {
          string str,str2="";
            int count=1,flag=0;
            str="HelloWorld";
            //int n=str.Length-1;
            //Console.WriteLine(n);
            for(int i=0;i<str.Length; i++)
            {
                foreach(char c in str2)
                {
                    if(c == str[i])
                    {
                        flag=1;
                        //continue;
                        break;
                    }
                   
                }
                
                 if (flag == 1)
                   {
                   // Console.Write("print");
                   continue;        
                   }
                for(int j=i+1;j<str.Length;j++)
                {
                    if (str[i] == str[j])
                    {
                        count+=1;
                        str2+=str[j];
                    }
                    else
                    {
                        
                    }
                   
                    
                }
                 Console.WriteLine("{0} count is{1}",str[i],count);
                count=1;
            
                
                
            }
        }
    }
}

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Compilation time: 0,14 sec, absolute running time: 0,11 sec, cpu time: 0,09 sec, average memory usage: 15 Mb, average nr of threads: 3