388. Longest Absolute File Path

Description

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:

file system

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:

file system

The directory dir contains two sub-directories subdir1 and subdir2. subdir1 contains a file file1.ext and an empty second-level sub-directory subsubdir1. subdir2 contains 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, return 0.

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.

Solution

Stack, time O(n), space O(n)

花了好一会儿才弄明白题意，\t是每行的开头，不是结尾……也是服了自己了。

首先将input按照"\n"分隔成一组tokens，对于每个token，"\t"的个数即为它所在的level。然后利用stack处理即可，先将不属于当前path的元素出栈，然后如果当前是file直接update longest path len，如果是directory则入栈。

class Solution {
    public int lengthLongestPath(String input) {
        int longest = 0;
        Stack stack = new Stack<>();   // store current path len
        int pathLen = 0;
        
        for (String token : input.split("\n")) {
            int[] pair = getLevelAndLen(token);
            int level = pair[0];
            int len = pair[1];
            
            while (stack.size() > level) {
                pathLen -= stack.pop();
            }
            
            if (token.contains(".")) {  // met a file, update longest
                longest = Math.max(longest, pathLen + len);
                continue;
            }
            
            ++len;              // adding a "/"
            stack.push(len);    // push directory into stack
            pathLen += len;     // add to current path length
        }
        
        return longest;
    }
    
    public int[] getLevelAndLen(String s) {
        String compact = s.replaceAll("\t", "");
        int[] res = new int[2];
        res[0] = s.length() - compact.length();
        res[1] = compact.length();
        return res;
    }
}