Find first repeating Char in String
Your checkOccurrence(s) is just a clumsy way to write s.find(countMap(_) > 1).

You can significantly simplify the solution by taking advantage of .distinct.

def firstUnique(s: String): Option[Char] =
  s.zipAll(s.distinct, '\u0000', '\u0000')
   .collectFirst({ case ab if ab._1 != ab._2 =&gt; ab._1 })