# Remove Sub-Directories from a File System

Given an array of strings arr[] consisting of N unique directories in the form of strings, the task is to remove all the subdirectories and print the final array.

A directory S is a subdirectory if there exists any directory D such that S is the prefix of D.

Examples:

Input: arr[] = {“/a”, “/a/j”, “/c/d/e”, “/c/d”, “/b”}
Output: { “/a”, “/c/d”, “/b”}
Explanation: Directory “/a/j” is a subdirectory of directory “/a”, and “/c/d/e” is a subdirectory of directory “/c/d”.
Therefore, remove both of them.

Input: arr[] = {“/a”, “/a/b”, “/a/b/c”, “/a/b/c/d”}
Output: {“/a”}
Explanation: All directories are subdirectories of “/a”

Naive Approach: The simplest approach is to traverse the array to check for each string, if there exists a string having it as the prefix and remove such strings from the array.

Time Complexity: O(len * N2), where len is the length of the longest string in the array.
Auxiliary Space: O(N)

Efficient Approach: The idea is to sort the given array and compare the current directory at position i with the previous directory at index (i -1). If arr[i – 1] is the prefix of arr[i] then remove arr[i]. After checking the whole array, print the remaining directories. Follow the below steps to solve the problem:

• Sort all the directories alphabetically and initialize a vector res.
• Insert the first directory into res.
• Check each directory with the last valid directory i.e., last directory from res.
• The directory is invalid(sub-directory) if it equals to some prefix of the previous valid directory. Else it is valid.
• If the current directory is valid, then add it to res.
• Print all the directories present in res after done traversing.

Below is the implementation of the above approach:

## C++

 `// C++ program for the above approach`   `#include ` `using` `namespace` `std;`   `// Function to remove sub-directories` `// from the given lists dir` `void` `eraseSubdirectory(vector& dir)` `{` `    ``// Store final result` `    ``vector res;`   `    ``// Sort the given directories` `    ``sort(dir.begin(), dir.end());`   `    ``// Insert 1st directory` `    ``res.push_back(dir[0]);`   `    ``cout << ``"{"` `<< dir[0] << ``", "``;`   `    ``// Iterating in directory` `    ``for` `(``int` `i = 1; i < dir.size(); i++) {`   `        ``// Current directory` `        ``string curr = dir[i];`   `        ``// Our previous valid directory` `        ``string prev = res.back();`   `        ``// Find length of previous directory` `        ``int` `l = prev.length();`   `        ``// If subdirectory is found` `        ``if` `(curr.length() > l && curr[l] == ``'/'` `            ``&& prev == curr.substr(0, l))` `            ``continue``;`   `        ``// Else store it in result` `        ``// valid directory` `        ``res.push_back(curr);`   `        ``cout << curr << ``", "``;` `    ``}`   `    ``cout << ``"}\n"``;` `}`   `// Driver Code` `int` `main()` `{` `    ``// Given lists of directories dir[]` `    ``vector dir` `        ``= { ``"/a"``, ``"/a/j"``, ``"/c/d/e"``,` `            ``"/c/d"``, ``"/b"` `};`   `    ``// Function Calll` `    ``eraseSubdirectory(dir);`   `    ``return` `0;` `}`

## Java

 `// Java program for the above approach` `import` `java.io.*;` `import` `java.util.*;`   `class` `GFG{`   `// Function to remove sub-directories` `// from the given lists dir` `static` `void` `eraseSubdirectory(ArrayList dir)` `{` `    `  `    ``// Store final result` `    ``ArrayList res = ``new` `ArrayList();`   `    ``// Sort the given directories` `    ``Collections.sort(dir);`   `    ``// Insert 1st directory` `    ``res.add(dir.get(``0``));`   `    ``System.out.print(``"{"` `+ dir.get(``0``) + ``", "``);`   `    ``// Iterating in directory` `    ``for``(``int` `i = ``1``; i < dir.size(); i++)` `    ``{` `        `  `        ``// Current directory` `        ``String curr = dir.get(i);`   `        ``// Our previous valid directory` `        ``String prev = res.get(res.size() - ``1``);`   `        ``// Find length of previous directory` `        ``int` `l = prev.length();`   `        ``// If subdirectory is found` `        ``if` `(curr.length() > l &&` `           ``curr.charAt(l) == ``'/'` `&& ` `           ``prev.equals(curr.substring(``0``, l)))` `            ``continue``;`   `        ``// Else store it in result` `        ``// valid directory` `        ``res.add(curr);`   `        ``System.out.print(curr + ``", "``);` `    ``}` `    ``System.out.print(``"}\n"``);` `}`   `// Driver Code` `public` `static` `void` `main(String[] args)` `{` `    `  `    ``// Given lists of directories dir[]` `    ``ArrayList dir = ``new` `ArrayList(Arrays.asList(` `            ``"/a"``, ``"/a/j"``, ``"/c/d/e"``, ``"/c/d"``, ``"/b"``));`   `    ``// Function Call` `    ``eraseSubdirectory(dir);` `}` `}`   `// This code is contributed by akhilsaini`

## Python3

 `# Python3 program for the above approach`   `# Function to remove sub-directories` `# from the given lists dir` `def` `eraseSubdirectory(``dir``):` `    `  `    ``# Store final result` `    ``res ``=` `[]`   `    ``# Sort the given directories` `    ``dir``.sort()`   `    ``# Insert 1st directory` `    ``res.append(``dir``[``0``])`   `    ``print``(``"{"``, ``dir``[``0``], end ``=` `", "``)`   `    ``# Iterating in directory` `    ``for` `i ``in` `range``(``1``, ``len``(``dir``)):` `        `  `        ``# Current directory` `        ``curr ``=` `dir``[i]`   `        ``# Our previous valid directory` `        ``prev ``=` `res[``len``(res) ``-` `1``]`   `        ``# Find length of previous directory` `        ``l ``=` `len``(prev)`   `        ``# If subdirectory is found` `        ``if` `(``len``(curr) > l ``and` `           ``curr[l] ``=``=` `'/'` `and` `              ``prev ``=``=` `curr[:l]):` `            ``continue`   `        ``# Else store it in result` `        ``# valid directory` `        ``res.append(curr)` `        ``print``(curr, end ``=` `", "``)`   `    ``print``(``"}"``)`   `# Driver Code` `if` `__name__ ``=``=` `'__main__'``:`   `    ``# Given lists of directories dir[]` `    ``dir` `=` `[ ``"/a"``, ``"/a/j"``, ``"/c/d/e"``, ``"/c/d"``, ``"/b"` `]`   `    ``# Function Call` `    ``eraseSubdirectory(``dir``)`   `# This code is contributed by akhilsaini`

## C#

 `// C# program for the above approach` `using` `System;` `using` `System.Collections;`   `class` `GFG{`   `// Function to remove sub-directories` `// from the given lists dir` `static` `void` `eraseSubdirectory(ArrayList dir)` `{` `    `  `    ``// Store final result` `    ``ArrayList res = ``new` `ArrayList();`   `    ``// Sort the given directories` `    ``dir.Sort();`   `    ``// Insert 1st directory` `    ``res.Add(dir[0]);`   `    ``Console.Write(``"{"` `+ dir[0] + ``", "``);`   `    ``// Iterating in directory` `    ``for``(``int` `i = 1; i < dir.Count; i++) ` `    ``{` `        `  `        ``// Current directory` `        ``string` `curr = (``string``)dir[i];`   `        ``// Our previous valid directory` `        ``string` `prev = (``string``)res[(res.Count - 1)];`   `        ``// Find length of previous directory` `        ``int` `l = prev.Length;`   `        ``// If subdirectory is found` `        ``if` `(curr.Length > l && curr[l] == ``'/'` `&&` `            ``prev.Equals(curr.Substring(0, l)))` `            ``continue``;`   `        ``// Else store it in result` `        ``// valid directory` `        ``res.Add(curr);`   `        ``Console.Write(curr + ``", "``);` `    ``}` `    ``Console.Write(``"}\n"``);` `}`   `// Driver Code` `public` `static` `void` `Main()` `{` `    `  `    ``// Given lists of directories dir[]` `    ``ArrayList dir = ``new` `ArrayList(){ ``"/a"``, ``"/a/j"``, ` `                                     ``"/c/d/e"``, ` `                                     ``"/c/d"``, ``"/b"` `};`   `    ``// Function Call` `    ``eraseSubdirectory(dir);` `}` `}`   `// This code is contributed by akhilsaini`

Output:

```{/a, /b, /c/d, }

```

Time Complexity: O(N*log N)
Auxiliary Space: O(N)

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