https://leetcode.com/problems/time-needed-to-inform-all-employees/description/
1376. Time Needed to Inform All Employees
給你數字 n 表示員工數量，headID 表示老闆，manager[] 表示每個人他的上司是誰，
informTime[] 表示通知下屬要花多久(沒下屬的話是0)，今天老闆有話要通知全公司
，他會通知地位最高的主管並讓主管交接給他的下級主管，題目保證所有員工都會被
通知，請問全公司都收到消息最快要花多久?
Example 1:
Input: n = 1, headID = 0, manager = [-1], informTime = [0]
Output: 0
Explanation: The head of the company is the only employee in the company.
Example 2:
Input: n = 6, headID = 2, manager = [2,2,-1,2,2,2], informTime = [0,0,1,0,0,0]
https://assets.leetcode.com/uploads/2020/02/27/graph.png
Output: 1
Explanation: The head of the company with id = 2 is the direct manager of all
the employees in the company and needs 1 minute to inform them all.
The tree structure of the employees in the company is shown.
思路：
1.所有員工都要被通知到，可以把關係轉變成一顆樹，老闆當作是子節點並記錄
每個人的下屬有哪些。
2.對所有下屬做DFS，並比較所有底層員工收到消息時花了多久，取最長的時間。
Java Code：