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.