https://leetcode.com/problems/minimum-number-of-vertices-to-reach-all-nodes/description/
1557. Minimum Number of Vertices to Reach All Nodes
找出最小的集合數量，需要滿足從每個集合的其中一個點出發，走過每個集合後可以到
達所有的節點。
Example 1:
https://assets.leetcode.com/uploads/2020/07/07/untitled22.png
Input: n = 6, edges = [[0,1],[0,2],[2,5],[3,4],[4,2]]
Output: [0,3]
Explanation: It's not possible to reach all the nodes from a single vertex.
From 0 we can reach [0,1,2,5]. From 3 we can reach [3,4,2,5]. So we output
[0,3].
Example 2:
https://assets.leetcode.com/uploads/2020/07/07/untitled.png
Input: n = 5, edges = [[0,1],[2,1],[3,1],[1,4],[2,4]]
Output: [0,2,3]
Explanation: Notice that vertices 0, 3 and 2 are not reachable from any other
node, so we must include them. Also any of these vertices can reach nodes 1
and 4.
思路：
1.建立無向圖
2.用dfs + 併查集把每個節點分組
3.把每個節點的root都放進Set
4.返回Set裡面的編號
Java Code：