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.

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.