String Compressor (Dima's version)

    fun compress2(s: String): String {
        val sb = StringBuilder()
        var l = 0
        var r = 0
        while (l < s.length) {
            while (r < s.length && s[l] == s[r]) r++
            sb.append(s[l])
            if(r - l > 1) sb.append(r - l)

            l = r
        }

        return sb.toString()
    }