1233. Remove Sub-Folders from the Filesystem.java

1233. Remove Sub-Folders from the Filesystem
Solved
Medium
Topics
premium lock icon
Companies
Hint
Given a list of folders folder, return the folders after removing all sub-folders in those folders. You may return the answer in any order.

If a folder[i] is located within another folder[j], it is called a sub-folder of it. A sub-folder of folder[j] must start with folder[j], followed by a "/". For example, "/a/b" is a sub-folder of "/a", but "/b" is not a sub-folder of "/a/b/c".

The format of a path is one or more concatenated strings of the form: '/' followed by one or more lowercase English letters.

For example, "/leetcode" and "/leetcode/problems" are valid paths while an empty string and "/" are not.
 

Example 1:

Input: folder = ["/a","/a/b","/c/d","/c/d/e","/c/f"]
Output: ["/a","/c/d","/c/f"]
Explanation: Folders "/a/b" is a subfolder of "/a" and "/c/d/e" is inside of folder "/c/d" in our filesystem.
Example 2:

Input: folder = ["/a","/a/b/c","/a/b/d"]
Output: ["/a"]
Explanation: Folders "/a/b/c" and "/a/b/d" will be removed because they are subfolders of "/a".
Example 3:

Input: folder = ["/a/b/c","/a/b/ca","/a/b/d"]
Output: ["/a/b/c","/a/b/ca","/a/b/d"]
 

Constraints:

1 <= folder.length <= 4 * 104
2 <= folder[i].length <= 100
folder[i] contains only lowercase letters and '/'.
folder[i] always starts with the character '/'.
Each folder name is unique


class Solution {

    public List<String> removeSubfolders(String[] folder) {
        // Create a set to store all folder paths for fast lookup
        Set<String> folderSet = new HashSet<>(Arrays.asList(folder));
        List<String> result = new ArrayList<>();

        // Iterate through each folder to check if it's a sub-folder
        for (String f : folder) {
            boolean isSubFolder = false;
            String prefix = f;

            // Check all prefixes of the current folder path
            while (!prefix.isEmpty()) {
                int pos = prefix.lastIndexOf('/');
                if (pos == -1) break;

                // Reduce the prefix to its parent folder
                prefix = prefix.substring(0, pos);

                // If the parent folder exists in the set, mark as sub-folder
                if (folderSet.contains(prefix)) {
                    isSubFolder = true;
                    break;
                }
            }

            // If not a sub-folder, add it to the result
            if (!isSubFolder) {
                result.add(f);
            }
        }

        return result;
    }
}