Given an array `arr` of 4 digits, find the latest 24-hour time that can be made using each digit exactly once.

24-hour times are formatted as `"HH:MM"`, where `HH` is between `00` and `23`, and `MM` is between `00` and `59`. The earliest 24-hour time is `00:00`, and the latest is `23:59`.

Return the latest 24-hour time in `"HH:MM"` format. If no valid time can be made, return an empty string.

Example 1:

    Input: arr = [1,2,3,4]
    Output: "23:41"
    Explanation: The valid 24-hour times are "12:34", "12:43", "13:24", "13:42", "14:23", "14:32", "21:34", "21:43", "23:14", and "23:41". Of these times, "23:41" is the latest.

Example 2:

    Input: arr = [5,5,5,5]
    Output: ""
    Explanation: There are no valid 24-hour times as "55:55" is not valid.

Constraints:

- `arr.length == 4`
- `0 <= arr[i] <= 9`