Write a program in Swift to find the first non-repeating character in a given string?

func firstNotRepeatingCharacter(s: String) -> Character {

    var occurrences: [Character:Int] = [:]

    s.forEach{ occurrences[$0] = (occurrences[$0] ?? 0) + 1}

    return s.first{ occurrences[$0] == 1 } ?? "_"

}