Решение

class Solution {
    fun convertToTitle(columnNumber: Int): String {
        var n = columnNumber
        val buf = CharArray(8)
        var len = 0
        while (n > 0) {
            n--
            buf[len++] = (65 + (n % 26)).toChar()
            n /= 26
        }
        val res = CharArray(len)
        var i = 0
        while (i < len) {
            res[i] = buf[len - 1 - i]
            i++
        }
        return String(res)
    }
}