题目：
Given an absolute path for a file (Unix-style), simplify it.
For example,
path = "/home/", => "/home"
path = "/a/./b/../../c/", => "/c"
click to show corner cases.
思路
unix style path的规则如下：
/ -> root
/a -> in (a)
. -> THIS dir path
/a/./ -> still in /a
/a/./b -> in /a/b
.. -> go "up" one level
/a/./b/.. -> /a/b/.. -> /a
/a/./b/../.. -> /a/.. -> /
/a/./b/../../c -> /c
解法
public class Solution {
public String simplifyPath(String path) {
Stack<String> stack = new Stack<String>();
//三种需要跳过的情况
Set<String> skip = new HashSet<String>(Arrays.asList("..", ".", ""));
for (String dir : path.split("/")) {
//当遇到..时，需要向前进
if (dir.equals("..") && !stack.isEmpty()) {
stack.pop();
} else if (!skip.contains(dir)) {
stack.push(dir);
}
}
String result = "";
if (stack.isEmpty()) result += "/";
while (!stack.isEmpty()) {
//pop出来的顺序是反的，所以加的时候，把最新pop出来的路径加在前面
result = "/" + stack.pop() + result;
}
return result;
}
}
