https://leetcode.com/problems/minimum-number-of-vertices-to-reach-all-nodes/description/
1557. Minimum Number of Vertices to Reach All Nodes
找出最小的集合数量，需要满足从每个集合的其中一个点出发，走过每个集合后可以到
达所有的节点。
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.建立无向图
2.用dfs + 并查集把每个节点分组
3.把每个节点的root都放进Set
4.返回Set里面的编号
Java Code：