# 71. Simplify Path (M)

Given a string `path`, which is an **absolute path** (starting with a slash `'/'`) to a file or directory in a Unix-style file system, convert it to the simplified **canonical path**.

In a Unix-style file system, a period `'.'` refers to the current directory, a double period `'..'` refers to the directory up a level, and any multiple consecutive slashes (i.e. `'//'`) are treated as a single slash `'/'`. For this problem, any other format of periods such as `'...'` are treated as file/directory names.

The **canonical path** should have the following format:

* The path starts with a single slash `'/'`.
* Any two directories are separated by a single slash `'/'`.
* The path does not end with a trailing `'/'`.
* The path only contains the directories on the path from the root directory to the target file or directory (i.e., no period `'.'` or double period `'..'`)

Return *the simplified **canonical path***.

 

**Example 1:**

```
Input: path = "/home/"
Output: "/home"
Explanation: Note that there is no trailing slash after the last directory name.
```

**Example 2:**

```
Input: path = "/../"
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: path = "/home//foo/"
Output: "/home/foo"
Explanation: In the canonical path, multiple consecutive slashes are replaced by a single one.
```

 

**Constraints:**

* `1 <= path.length <= 3000`
* `path` consists of English letters, digits, period `'.'`, slash `'/'` or `'_'`.
* `path` is a valid absolute Unix path.

### Solution:

利用栈先进后出的特性处理上级目录 `..`，最后组装化简后的路径即可。

```
class Solution {
    public String simplifyPath(String path) {
        String[] parts = path.split("/");
        Stack<String> stk = new Stack<>();
        // 借助栈计算最终的文件夹路径
        for (String part : parts) {
            if (part.isEmpty() || part.equals(".")) {
                continue;
            }
            if (part.equals("..")) {
                if (!stk.isEmpty()) stk.pop();
                continue;
            }
            stk.push(part);
        }
        // 栈中存储的文件夹组成路径
        String res = "";
        while (!stk.isEmpty()) {
            res = "/" + stk.pop() + res;
        }
        return res.isEmpty() ? "/" : res;
    }
}
```