func isPangram(str: String) -> String {
    //if not missing letters return ""
    var alphabetSet = Set("qwertyuiopasdfghjklzxcvbnm")
    var inputStrSet = Set(str.lowercased())
    var missingLetters = String(alphabetSet.subtracting(inputStrSet))
    print(missingLetters)
    if missingLetters == "" {return ""}
    missingLetters.count
    return missingLetters
}
isPangram(str: "the quick brown fox jumps over the lazy dog")