<p>Given an array <code>arr</code> of 4 digits, find the latest 24-hour time that can be made using each digit <strong>exactly once</strong>.</p> <p>24-hour times are formatted as <code>"HH:MM"</code>, where <code>HH</code> is between <code>00</code> and <code>23</code>, and <code>MM</code> is between <code>00</code> and <code>59</code>. The earliest 24-hour time is <code>00:00</code>, and the latest is <code>23:59</code>.</p> <p>Return <em>the latest 24-hour time in <code>"HH:MM"</code> format</em>. If no valid time can be made, return an empty string.</p> <p> </p> <p><strong class="example">Example 1:</strong></p> <pre> <strong>Input:</strong> arr = [1,2,3,4] <strong>Output:</strong> "23:41" <strong>Explanation:</strong> 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. </pre> <p><strong class="example">Example 2:</strong></p> <pre> <strong>Input:</strong> arr = [5,5,5,5] <strong>Output:</strong> "" <strong>Explanation:</strong> There are no valid 24-hour times as "55:55" is not valid. </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li><code>arr.length == 4</code></li> <li><code>0 <= arr[i] <= 9</code></li> </ul>