Решение

class Solution {
    fun maximumTime(time: String): String {
        val arr = time.toCharArray()
        if (arr[0] == '?') {
            arr[0] = if (arr[1] != '?' && arr[1] > '3') '1' else '2'
        }
        if (arr[1] == '?') {
            arr[1] = if (arr[0] == '2') '3' else '9'
        }
        if (arr[3] == '?') arr[3] = '5'
        if (arr[4] == '?') arr[4] = '9'
        return String(arr)
    }
}