using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(ReplaceDigits("6531"));

    }

    public static int EndIndexOf(string source, string value)

{

    int index = source.IndexOf(value);

    if (index >= 0 && index< source.Length)

    {

        index += value.Length;

    }

​

    return index;

}

        public static string ReplaceDigits(string sentence)

        {

            string[] words = { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };

            for (var i=0; i < words.Length; i++)

            {

                sentence = sentence.Replace(i.ToString(), words[i]);

            }

            foreach(var w in words)

            {

                int a=EndIndexOf(sentence,w);

                if(a<sentence.Length-1)

                {

                    a=a+1;

                }

                if(!char.IsWhiteSpace(sentence[a]))

                {

                    sentence = sentence.Replace(w, w+" ");

                }

            }

            return sentence;

        }

}