<p>You are given a string <code>num</code> consisting of digits only.</p> <p>Return <em>the <strong>largest palindromic</strong> integer (in the form of a string) that can be formed using digits taken from </em><code>num</code>. It should not contain <strong>leading zeroes</strong>.</p> <p><strong>Notes:</strong></p> <ul> <li>You do <strong>not</strong> need to use all the digits of <code>num</code>, but you must use <strong>at least</strong> one digit.</li> <li>The digits can be reordered.</li> </ul> <p> </p> <p><strong class="example">Example 1:</strong></p> <pre> <strong>Input:</strong> num = "444947137" <strong>Output:</strong> "7449447" <strong>Explanation:</strong> Use the digits "4449477" from "<u><strong>44494</strong></u><u><strong>7</strong></u>13<u><strong>7</strong></u>" to form the palindromic integer "7449447". It can be shown that "7449447" is the largest palindromic integer that can be formed. </pre> <p><strong class="example">Example 2:</strong></p> <pre> <strong>Input:</strong> num = "00009" <strong>Output:</strong> "9" <strong>Explanation:</strong> It can be shown that "9" is the largest palindromic integer that can be formed. Note that the integer returned should not contain leading zeroes. </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li><code>1 <= num.length <= 10<sup>5</sup></code></li> <li><code>num</code> consists of digits.</li> </ul>