Longest Absolute File Path

Suppose we have a file system that stores both files and directories. An example of one system is represented in the following picture:

Here, we have dir as the only directory in the root. dir contains two subdirectories, subdir1 and subdir2. subdir1 contains a file file1.ext and subdirectory subsubdir1. subdir2 contains a subdirectory subsubdir2, which contains a file file2.ext.

In text form, it looks like this (with ⟶ representing the tab character):

dir
⟶ subdir1
⟶ ⟶ file1.ext
⟶ ⟶ subsubdir1
⟶ subdir2
⟶ ⟶ subsubdir2
⟶ ⟶ ⟶ file2.ext

If we were to write this representation in code, it will look like this: "dir\n\tsubdir1\n\t\tfile1.ext\n\t\tsubsubdir1\n\tsubdir2\n\t\tsubsubdir2\n\t\t\tfile2.ext". Note that the '\n' and '\t' are the new-line and tab characters.

Every file and directory has a unique absolute path in the file system, which is the order of directories that must be opened to reach the file/directory itself, all concatenated by '/'s. Using the above example, the absolute path to file2.ext is "dir/subdir2/subsubdir2/file2.ext". Each directory name consists of letters, digits, and/or spaces. Each file name is of the form name.extension, where name and extension consist of letters, digits, and/or spaces.

Given a string input representing the file system in the explained format, return the length of the longest absolute path to a file in the abstracted file system. If there is no file in the system, return 0.

Example 1:

Input: input = "dir\n\tsubdir1\n\tsubdir2\n\t\tfile.ext"
Output: 20
Explanation: We have only one file, and the absolute path is "dir/subdir2/file.ext" of length 20.

Example 2:

Input: input = "dir\n\tsubdir1\n\t\tfile1.ext\n\t\tsubsubdir1\n\tsubdir2\n\t\tsubsubdir2\n\t\t\tfile2.ext"
Output: 32
Explanation: We have two files:
"dir/subdir1/file1.ext" of length 21
"dir/subdir2/subsubdir2/file2.ext" of length 32.
We return 32 since it is the longest absolute path to a file.

Example 3:

Input: input = "a"
Output: 0
Explanation: We do not have any files, just a single directory named "a".

Example 4:

Input: input = "file1.txt\nfile2.txt\nlongfile.txt"
Output: 12
Explanation: There are 3 files at the root directory.
Since the absolute path for anything at the root directory is just the name itself, the answer is "longfile.txt" with length 12.

Constraints:

1 <= input.length <= 104
input may contain lowercase or uppercase English letters, a new line character '\n', a tab character '\t', a dot '.', a space ' ', and digits.

Solution:

keep track of a list of current directories, update the list if we see less tabs than the length of list

class Solution {
    /*
    dir
    \n\tsubdir1
    \n\t\tfile1.ext
    \n\t\tsubsubdir1
    \n\tsubdir2
    \n\t\tsubsubdir2
    \n\t\t\tfile2.ext 
    */
    public int lengthLongestPath(String input) {
        List<String> dirs = new ArrayList();
        int max = 0;
        int prevtabs = 0;
        for (int i = 0; i < input.length();) {
            int istart = i;
            int tabs = 0;
            while (i < input.length() && input.charAt(i) != '\n') {
                i ++;
            }
            String file = input.substring(istart, i);
            i ++;
            while (i < input.length() && input.charAt(i) == '\t') {
                i ++;
                tabs ++;
            }
            if (dirs.size() > 0 && prevtabs < dirs.size()) {
                dirs = dirs.subList(0, prevtabs);
            }
            // System.out.println(file + ", " + dirs + ", " + prevtabs);
            if (file.contains(".")) {
                int dirLen = 0;
                for (String dir : dirs) {
                    dirLen += dir.length();
                }
                max = Math.max(max, file.length() + dirLen + dirs.size());
            } else {
                dirs.add(file);
            }
            prevtabs = tabs;
        }
        return max;
    }
}