71. 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"

难度：medium

题目：给出unix风格的绝对路径，简化它。换名话说转成canonical 路径。 在unix风格系统里。.指当前目录。..指上一级目录。

思路：stack

Runtime: 14 ms, faster than 74.20% of Java online submissions for Simplify Path.
Memory Usage: 37.2 MB, less than 1.00% of Java online submissions for Simplify Path.

class Solution {
    public String simplifyPath(String path) {
        String[] strs = path.split("/");
        Stack<String> stack = new Stack<String>();
        
        for (int i = 0; i < strs.length; i++) {
            if (strs[i].isEmpty() || strs[i].equals(".")) {
                continue;
            }
            
            if (strs[i].equals("..")) {
                if (!stack.isEmpty()) stack.pop();
            } else {
                stack.push("/" + strs[i]);
            }
        }
        
        StringBuilder sb = new StringBuilder();
        for (String s: new ArrayList<String>(stack)) {
            sb.append(s);
        }
        
        return stack.isEmpty() ? "/" : sb.toString();
    }
}