Find and replace number with string using C#

Find and replace number with string using C#

Input: "1 2 are working in the 3 company"

Output :"one two are working in the three company"

using System;

using System.Text;

using System.Collections.Generic;

namespace Coding.Exercise

{

    public class Exercise

    {

                // TODO: fix this method - fix bugs, make more efficient, and return correct result

        public static string ReplaceDigits(string sentence)

        {

          // int length =sentence.Length;

           StringBuilder  sb = new StringBuilder ();

           //string result =sentence;

char[] characters = sentence.ToCharArray();

for(int i =0; i<characters.Length;i++ )

{

char a =characters[i];

Console.WriteLine(characters[i]);

if(char.IsDigit(a))

{ 

   // Console.WriteLine(characters[i+1]);

    if(i+1<characters.Length && ! char.IsWhiteSpace(characters[i+1]))

    {

        sb.Append(ReturnNumberString(a)+" ");  

    }

    else

    {

     sb.Append(ReturnNumberString(a)); 

    }

    

          //result=result.Replace(Convert.ToString(a),ReturnNumberString(a));

         // Console.WriteLine(result);

}

else{

    sb.Append(characters[i]);

}

}

  return sb.ToString();

        }

        

      public static string ReturnNumberString(char number)

        {

            switch(number)

            {

            case '1': return "one" ;break;    

            case '2': return "two"; break; 

            case '3': return "three"; break; 

            case '4': return "four"; break; 

            case '5': return "five"; break; 

            case '6': return "six"; break; 

            case '7': return "seven"; break; 

            case '8':return  "eight"; break; 

            case '9':return  "nine"; break; 

            case '0':return  "zero "; break; 

            default: return"";break;

            }

            

        }

    }

}