[문제풀이] 신규 아이디 추천

    fun recommendationNewId(newId: String): String = newId
        .toLowerCase()
        .filter {
            it in '0'..'9' || it in charArrayOf('_', '-', '.') || it in 'a'..'z'
        }
        .let { step3(it) }
        .let { step4(it) }
        .let(step5)
        .let(step6)
        .let(step7)

    fun step3(id: String): String {
        val temp = id.replace("..", ".")
        if (!temp.contains("..")) {
            return temp
        }
        return step3(temp)
    }

    fun step4(id: String): String {
        var temp = if (id.first() == '.') id.drop(1) else id
        temp = if (temp.isNotEmpty() && temp.last() == '.') temp.dropLast(1) else temp

        if (temp.isEmpty() || (temp.first() != '.' && temp.last() != '.')) {
            return temp
        }

        return step4(temp)
    }

    val step5 = { id: String ->
        if (id.isBlank() || id.isEmpty()) "a" else id
    }

    val step6 = { id: String ->
        if (id.length > 15)
            step4(id.substring(0, 15))
        else
            id
    }

    val step7 = { id: String ->
        var temp = id
        while (temp.length < 3) {
            temp = temp.plus(id.last())
        }
        temp
    }


    // 우수 답안

    class Solution {
        fun solution(newId: String) = newId.toLowerCase()
            .filter { it.isLowerCase() || it.isDigit() || it == '-' || it == '_' || it == '.' }
            .replace("[.]*[.]".toRegex(), ".")
            .removePrefix(".").removeSuffix(".")
            .let { if (it.isEmpty()) "a" else it }
            .let { if (it.length > 15) it.substring(0 until 15) else it }.removeSuffix(".")
            .let {
                if (it.length <= 2)
                    StringBuilder(it).run {
                        while (length < 3) append(it.last())
                        toString()
                    }
                else it }
    }