compute recursively a
new string where adjacent characters that
are the same have been reduced to a single
character. So "yyzzza" yields "yza".
You must not use loops.
Here are some more examples:
string_clean("yyzzza") returns "yza"
string_clean("abbbcdd") returns "abcd"
string_clean("Hello") returns "Helo"
please give me some suggestion.