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 * 10^4
• 2 <= folder[i].length <= 100
• folder[i] contains only lowercase letters and ‘/’
• folder[i] always starts with character ‘/’
• Each folder name is unique.

Solution: HashTable

Time complexity: O(n*L)
Space complexity: O(n*L)

// Author: Huahua
class Solution {
public:
  vector<string> removeSubfolders(vector<string>& folder) {
    unordered_set<string> s(begin(folder), end(folder));
    vector<string> ans;
    for (const auto& cur : folder) {
      string f = cur;
      bool valid = true;
      while (!f.empty() && valid) {
        while (f.back() != '/') f.pop_back();
        f.pop_back();
        if (s.count(f)) valid = false;
      }
      if (valid) ans.push_back(cur);
    }
    return ans;
  }
};

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