string[] text = textBoxInput.Text.Split(' ');
List<string> replacements = new List<string>();

for(int i = 0; i < text.Length; i++)
{
    if(replacements.Contains(text[i]))
    {
        text[i] = ">" + replacements.IndexOf(text[i]).ToString() + "<";
    }
    else
    {
        replacements.Add(text[i]);
        text[i] = "(" + replacements.IndexOf(text[i]) + ")" + text[i];
    }
}

textBoxOutput.Text = String.Join(" ", text);