<p>Given a list of folders <code>folder</code>, return <em>the folders after removing all <strong>sub-folders</strong> in those folders</em>. You may return the answer in <strong>any order</strong>.</p> <p>If a <code>folder[i]</code> is located within another <code>folder[j]</code>, it is called a <strong>sub-folder</strong> of it.</p> <p>The format of a path is one or more concatenated strings of the form: <code>'/'</code> followed by one or more lowercase English letters.</p> <ul> <li>For example, <code>"/leetcode"</code> and <code>"/leetcode/problems"</code> are valid paths while an empty string and <code>"/"</code> are not.</li> </ul> <p> </p> <p><strong>Example 1:</strong></p> <pre> <strong>Input:</strong> folder = ["/a","/a/b","/c/d","/c/d/e","/c/f"] <strong>Output:</strong> ["/a","/c/d","/c/f"] <strong>Explanation:</strong> Folders "/a/b" is a subfolder of "/a" and "/c/d/e" is inside of folder "/c/d" in our filesystem. </pre> <p><strong>Example 2:</strong></p> <pre> <strong>Input:</strong> folder = ["/a","/a/b/c","/a/b/d"] <strong>Output:</strong> ["/a"] <strong>Explanation:</strong> Folders "/a/b/c" and "/a/b/d" will be removed because they are subfolders of "/a". </pre> <p><strong>Example 3:</strong></p> <pre> <strong>Input:</strong> folder = ["/a/b/c","/a/b/ca","/a/b/d"] <strong>Output:</strong> ["/a/b/c","/a/b/ca","/a/b/d"] </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li><code>1 <= folder.length <= 4 * 10<sup>4</sup></code></li> <li><code>2 <= folder[i].length <= 100</code></li> <li><code>folder[i]</code> contains only lowercase letters and <code>'/'</code>.</li> <li><code>folder[i]</code> always starts with the character <code>'/'</code>.</li> <li>Each folder name is <strong>unique</strong>.</li> </ul>