Simplify Path

Given an absolute path for a file (Unix-style), simplify it. Or in other words, convert it to the canonical path.

In a UNIX-style file system, a period . refers to the current directory. Furthermore, a double period … moves the directory up a level. For more information, see: Absolute path vs relative path in Linux/Unix

Note that the returned canonical path must always begin with a slash /, and there must be only a single slash / between two directory names. The last directory name (if it exists) must not end with a trailing /. Also, the canonical path must be the shortest string representing the absolute path.

Example 1:

Input: “/home/”
Output: “/home”
Explanation: Note that there is no trailing slash after the last directory name.
Example 2:

Input: “/…/”
Output: “/”
Explanation: Going one level up from the root directory is a no-op, as the root level is the highest level you can go.
Example 3:

Input: “/home//foo/”
Output: “/home/foo”
Explanation: In the canonical path, multiple consecutive slashes are replaced by a single one.
Example 4:

Input: “/a/./b/…/…/c/”
Output: “/c”
Example 5:

Input: “/a/…/…/b/…/c//.//”
Output: “/c”
Example 6:

Input: “/a//b////c/d//././/…”
Output: “/a/b/c”

解析：
通過棧模擬目錄返回

class Solution {
    public String simplifyPath(String path) {
         Stack<String> dirs = new Stack<>();

        for (int i = 0; i < path.length();) {
            ++i;

            int j = path.indexOf('/', i);
            if (j < 0) j = path.length();
            final String dir = path.substring(i, j);

            // 當有連續 '///'時，dir 爲空
            if (!dir.isEmpty() && !dir.equals(".")) {
                if (dir.equals("..")) {
                    if (!dirs.isEmpty())
                        dirs.pop();
                } else {
                    dirs.push(dir);
                }
            }

            i = j;
        }

        StringBuilder result = new StringBuilder();
        if (dirs.isEmpty()) {
            result.append('/');
        } else {
            for (final String dir : dirs) {
                result.append('/').append(dir);
            }
        }
        return result.toString();
    }
}