Longest Absolute File Path

Suppose we abstract our file system by a string in the following manner:

The string "dir\n\tsubdir1\n\tsubdir2\n\t\tfile.ext" represents:

dir
    subdir1
    subdir2
        file.ext

The directory dir contains an empty sub-directory subdir1 and a sub-directory subdir2 containing a file file.ext.

The string "dir\n\tsubdir1\n\t\tfile1.ext\n\t\tsubsubdir1\n\tsubdir2\n\t\tsubsubdir2\n\t\t\tfile2.ext" represents:

dir
    subdir1
        file1.ext
        subsubdir1
    subdir2
        subsubdir2
            file2.ext

The directory dir contains two sub-directories subdir1 and subdir2. subdir1 contains a file file1.ext and an empty second-level sub-directory subsubdir1. subdir2contains a second-level sub-directory subsubdir2 containing a file file2.ext.

We are interested in finding the longest (number of characters) absolute path to a file within our file system. For example, in the second example above, the longest absolute path is"dir/subdir2/subsubdir2/file2.ext", and its length is 32 (not including the double quotes).

Given a string representing the file system in the above format, return the length of the longest absolute path to file in the abstracted file system. If there is no file in the system, return0.

Note:

• 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.

The idea is just calculating the length of the absolute path of each file. We split the string by "\n". Then for each string, if we see a "\t", we know its a new depth, so we remove the tab and increment the depth. If we see "." in the string, we know it's a file, we calculate the path by adding its parent directory length to itself. If it's not a file, we record the length using a list, and by adding its parent directory length to itself. Since the directory is ordered, we go deep to the first file, then return back to another directory, like DFS. So overwritten a dir will not impact the total length.

public int lengthLongestPath(String input) {
        if (input.length() == 0) {
            return 0;
        }
        int maxLen = 0;
        String[] dirs = input.split("\\n");
        List depths = new ArrayList<>();
        depths.add(0);
        for (String line : dirs) {
            int depth = 0;
            while (line.charAt(0) == '\t') {
                line = line.substring(1);
                depth++;
            }
            if (line.indexOf('.') > 0) {
                maxLen = Math.max(maxLen, line.length() + depths.get(depth));
            } else {
                if (depth <= depths.size() - 2) {
                    depths.set(depth + 1, depths.get(depth) + line.length() + 1);
                } else {
                    depths.add(depths.get(depth) + line.length() + 1);
                }
            }
        }
        return maxLen;
    }