【Leetcode】Longest Absolute File Path

题目链接：https://leetcode.com/problems/longest-absolute-file-path/

题目：

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, 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.

思路：

思路比较简单，用一个可变数组维护一个从根目录到当前目录的绝对路径的长度。。。好绕口。。= =说出来自己都不懂。。

要注意绝对路径结尾必须包含一个文件才有算有效，且绝对路径需要一个长度为1的分隔符 \ 。

举个栗子：dir \n\tdir1 \n\tdir20 \n\t\tfile.ext  意思是：dir目录下有两文件夹dir1和dir20，其中dir20目录下有文件file.ext。

对input按从头开始顺序遍历。遇到dir，则当前数组：3；遇到dir1，则当前数组：3->(3+4),；遇到dir20，更新第2个元素，数组：3->(3+5)，遇到tfile.ext 数组：3->8->(8+9) 则最长绝对路径为dir dir20 tfile.ext = 17 ，最后因为目录之间需要一个 \ 分隔符，这里要加2个分隔符变为：dir\dir20\ tfile.ext 最长绝对路径为17+2=19。

算法：

public int lengthLongestPath(String input) {List<Integer> lenOfLevel = new ArrayList<Integer>();int max = 0, level = 0, preIdx = 0;input = input + '\n';//使得每个路径token后都接一个\n，便于统一处理boolean isFile = false;for (int i = 0; i < input.length(); i++) {if (input.charAt(i) == '.') {// 当前路径是否包含文件isFile = true;}if (input.charAt(i) != '\n') // 换一行continue;int tokenLen = i - preIdx + 1;//一个目录token的长度int tmp = tokenLen;if (level - 1 >= 0) {// 如果当前路径有上一层tmp += lenOfLevel.get(level - 1);// 则绝对路径长度需要加上上一层绝对路径长度}if (level <= lenOfLevel.size() - 1)// 当前路径有比它更深层的路径lenOfLevel.set(level, tmp);elselenOfLevel.add(tmp);// 当前路径是目前遇到的最深的路径if (isFile) {// 当前绝对路径包含文件，则是一条可以计算长度的有效路径max = Math.max(max, tmp);// 更新maxisFile = false;}int nIdx = i;// 更新待计算token的leveli++;while (i < input.length() && input.charAt(i) == '\t') {i++;}level = i - nIdx - 1;preIdx = i;// 更新待计算token的起始位置}return max - 1 >= 0 ? max - 1 : 0;}