Given a number K, we would want to reformat the strings such that each group contains exactly K characters, except for the first group which could be shorter than K, but still must contain at least one character. Furthermore, there must be a dash inserted between two groups and all lowercase letters should be converted to uppercase.

Code

fun licenseKeyFormatting(S: String, K: Int): String {
    val stringBuilder: StringBuilder = StringBuilder()
    var count = 0
    for (i in S.length - 1 downTo 0) {
      if (S[i] != '-') {
        if (count == K) {
          stringBuilder.append("-")
          count = 0
        }
        stringBuilder.append(S[i].toUpperCase())
        count++
      }
    }

    return stringBuilder.reverse().toString()
}