Replace repeated char with something else in a string

Question

I need help to figure out a logic:

So, let's say I have a string, and whenever there is a Char repeated inside that string, I need to replace it with a (Char + sequence of number).

For example:

Original String: "abcdefgabfabc"

Expected output: "abcdefga2b2f2a3b3c2"

'a' occurs 3 times, so the first 'a' remains as 'a', but the second 'a' becomes 'a2', and the third 'a' becomes 'a3' and the same goes to other chars like b, b2, b3...

Answer 1

1. Create a Dictionary of letters and the # of occurrences of each letter
2. Create a StringBuilder to store the output
3. Loop through the input string letter by letter
4. Output the letter to the new string
5. If a letter is not in the dictionary, add it as the key with a '1' as the value
6. If a letter is already in the dictionary, increase the value by 1 and append the value to the output string

Answer 2

Try this:

var foundChars = new SortedDictionary<char, int>();
var stringBuilder = new StringBuilder();

foreach (var c in originalString)
{
    var count = 0;

    if (!foundChars.TryGetValue(c, out count)
    {
        foundChars.Add(c, 1);
    }
    else
    {
        count += 1;
        foundChars[c] = count;
    }

    stringBuilder.Append(c);
    if (count > 0) stringBuilder.Append(count);
}

Note that while less pretty this will be more performant than LINQ based solutions and be retrocompatible with .NET 2.0.