For a undirected graph with tree characteristics, we can choose any node as the root. The result graph is then a rooted tree. Among all possible rooted trees, those with minimum height are called minimum height trees (MHTs). Given such a graph, write a function to find all the MHTs and return a list of their root labels.

Format

The graph contains n nodes which are labeled from 0 to n – 1. You will be given the number n and a list of undirected edges (each edge is a pair of labels).

You can assume that no duplicate edges will appear in edges. Since all edges are undirected, [0, 1] is the same as [1, 0] and thus will not appear together in edges.

Example 1:
Given n = 4, edges = [[1, 0], [1, 2], [1, 3]]
return `[1]`

**Example 2:**
Given `n = 6`

, `edges = [[0, 3], [1, 3], [2, 3], [4, 3], [5, 4]]`

return ```
[3, 4]
```

http://www.cnblogs.com/grandyang/p/5000291.html

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