• The name of a file contains at least a . and an extension.
• The name of a directory or sub-directory will not contain a .
• Time complexity required: O(n) where n is the size of the input string.
• Notice that a/aa/aaa/file1.txt is not the longest file path, if there is another path aaaaaaaaaaaaaaaaaaaaa/sth.png.

This problem is a straightforward string simulation algorithm problem. 

path[i] stores the prefix dir length for level i. path[0] is 0 since level 0 does not have any prefix;

for example string "dir\n\tsubdir1\n\tsubdir2\n\t\tfile.ext", path[1] is 4(representing dir/ as level 1‘s prefix).

1. split the input string around "\n" to get all dirs‘ and files‘ relative paths to their current level.

2. replace all "\t" with empty character "" for each substring and calcuate its level by checking how much shorter

the new substring is after the replace operation.

3. if the substring is a file, then add its prefix dir length and update the max absolute file path length if needed.

if not a file, update the prefix dir length for the next level.

4. each time the logic goes back to a higher level, it overwrites the stale prefix length values.

For example, "dir\n\tsubdir1\n\tsubdir\n\t\tfile.ext"

when processing substring "subdir2", path[0] = 0, path[1] = 4(dir/), path[2] = 12(dir/subdir1/), 

path[2] gets overwrite to 11(dir/subdir/). 

 1 public class Solution {
 2     public int lengthLongestPath(String input) {
 3         int result = 0;
 4         int[] path = new int[input.length() + 2];
 5         String[] st = input.split("\n");
 6         for (String line : st) {
 7             String name = line.replaceAll("(\t)+", "");
 8             int depth = line.length() - name.length();
 9             if(name.contains(".")) {
10                 result = Math.max(result, path[depth] + name.length());
11             }
12             else {
13                 path[depth + 1] = path[depth] + name.length() + 1;
14             }
15         }
16         return result;
17     }
18 }