fun caesarCipher(s: String, k: Int): String {
    return buildString{
        s.map{ letter ->
            when {
                letter in 'a'..'z' -> append('a' + (letter - 'a' + k) % 26) 
                letter in 'A'..'Z' -> append('A' + (letter - 'A' + k) % 26) 
                else -> append(letter)
            }
        }
    }
}