609.find-duplicate-file-in-system.md

## 题目地址
https://leetcode.com/problems/find-duplicate-file-in-system/description/

## 题目描述

```
Given a list of directory info including directory path, and all the files with contents in this directory, you need to find out all the groups of duplicate files in the file system in terms of their paths.

A group of duplicate files consists of at least two files that have exactly the same content.

A single directory info string in the input list has the following format:

"root/d1/d2/.../dm f1.txt(f1_content) f2.txt(f2_content) ... fn.txt(fn_content)"

It means there are n files (f1.txt, f2.txt ... fn.txt with content f1_content, f2_content ... fn_content, respectively) in directory root/d1/d2/.../dm. Note that n >= 1 and m >= 0. If m = 0, it means the directory is just the root directory.

The output is a list of group of duplicate file paths. For each group, it contains all the file paths of the files that have the same content. A file path is a string that has the following format:

"directory_path/file_name.txt"

Example 1:

Input:
["root/a 1.txt(abcd) 2.txt(efgh)", "root/c 3.txt(abcd)", "root/c/d 4.txt(efgh)", "root 4.txt(efgh)"]
Output:  
[["root/a/2.txt","root/c/d/4.txt","root/4.txt"],["root/a/1.txt","root/c/3.txt"]]
 

Note:

No order is required for the final output.
You may assume the directory name, file name and file content only has letters and digits, and the length of file content is in the range of [1,50].
The number of files given is in the range of [1,20000].
You may assume no files or directories share the same name in the same directory.
You may assume each given directory info represents a unique directory. Directory path and file info are separated by a single blank space.
 

Follow-up beyond contest:

1. Imagine you are given a real file system, how will you search files? DFS or BFS?

2. If the file content is very large (GB level), how will you modify your solution?

3. If you can only read the file by 1kb each time, how will you modify your solution?

4. What is the time complexity of your modified solution? What is the most time-consuming part and memory consuming part of it? How to optimize?

5. How to make sure the duplicated files you find are not false positive?

```

## 思路
思路就是hashtable去存储，key为文件内容，value为fullfilename，
遍历一遍去填充hashtable， 最后将hashtable中的值打印出来即可。

> 当且仅当有重复内容，我们才打印，因此我们需要过滤一下， 类似 filter(q => q.length >= 2)
## 关键点解析

- hashtable

## 代码
```js


/*
 * @lc app=leetcode id=609 lang=javascript
 *
 * [609] Find Duplicate File in System
 *
 * https://leetcode.com/problems/find-duplicate-file-in-system/description/
 *
 * algorithms
 * Medium (54.21%)
 * Total Accepted:    24.1K
 * Total Submissions: 44.2K
 * Testcase Example:  '["root/a 1.txt(abcd) 2.txt(efgh)","root/c 3.txt(abcd)","root/c/d 4.txt(efgh)","root 4.txt(efgh)"]'
 *
 * Given a list of directory info including directory path, and all the files
 * with contents in this directory, you need to find out all the groups of
 * duplicate files in the file system in terms of their paths.
 *
 * A group of duplicate files consists of at least two files that have exactly
 * the same content.
 *
 * A single directory info string in the input list has the following format:
 *
 * "root/d1/d2/.../dm f1.txt(f1_content) f2.txt(f2_content) ...
 * fn.txt(fn_content)"
 *
 * It means there are n files (f1.txt, f2.txt ... fn.txt with content
 * f1_content, f2_content ... fn_content, respectively) in directory
 * root/d1/d2/.../dm. Note that n >= 1 and m >= 0. If m = 0, it means the
 * directory is just the root directory.
 *
 * The output is a list of group of duplicate file paths. For each group, it
 * contains all the file paths of the files that have the same content. A file
 * path is a string that has the following format:
 *
 * "directory_path/file_name.txt"
 *
 * Example 1:
 *
 *
 * Input:
 * ["root/a 1.txt(abcd) 2.txt(efgh)", "root/c 3.txt(abcd)", "root/c/d
 * 4.txt(efgh)", "root 4.txt(efgh)"]
 * Output:
 *
 * [["root/a/2.txt","root/c/d/4.txt","root/4.txt"],["root/a/1.txt","root/c/3.txt"]]
 *
 *
 *
 *
 * Note:
 *
 *
 * No order is required for the final output.
 * You may assume the directory name, file name and file content only has
 * letters and digits, and the length of file content is in the range of
 * [1,50].
 * The number of files given is in the range of [1,20000].
 * You may assume no files or directories share the same name in the same
 * directory.
 * You may assume each given directory info represents a unique directory.
 * Directory path and file info are separated by a single blank space.
 *
 *
 *
 * Follow-up beyond contest:
 *
 *
 * Imagine you are given a real file system, how will you search files? DFS or
 * BFS?
 * If the file content is very large (GB level), how will you modify your
 * solution?
 * If you can only read the file by 1kb each time, how will you modify your
 * solution?
 * What is the time complexity of your modified solution? What is the most
 * time-consuming part and memory consuming part of it? How to optimize?
 * How to make sure the duplicated files you find are not false positive?
 *
 *
 */
/**
 * @param {string[]} paths
 * @return {string[][]}
 */
var findDuplicate = function(paths) {
  const hashmap = {};

  for (let path of paths) {
    const [folder, ...files] = path.split(" ");
    for (let file of files) {
      const lpi = file.indexOf("(");
      const rpi = file.lastIndexOf(")");
      const filename = file.slice(0, lpi);
      const content = file.slice(lpi, rpi);
      const fullname = `${folder}/${filename}`;
      if (!hashmap[content]) hashmap[content] = [];
      hashmap[content].push(fullname);
    }
  }

  return Object.values(hashmap).filter(q => q.length >= 2);
};
```

## 扩展
leetcode官方给的扩展我觉得就很有意思，虽然很`老套`, 这里还是列一下好了，大家可以作为思考题来思考一下。

1. Imagine you are given a real file system, how will you search files? DFS or BFS?

2. If the file content is very large (GB level), how will you modify your solution?

3. If you can only read the file by 1kb each time, how will you modify your solution?

4. What is the time complexity of your modified solution? What is the most time-consuming part and memory consuming part of it? How to optimize?

5. How to make sure the duplicated files you find are not false positive?