摘要：可以从头边同时进行，查看叶子节点并加入到叶子节点链表遍历一遍后，叶子节点链表为。将叶子节点保存下来。这个时候就会有第二层叶子节点那些在列表当中为的点，用同样的方法进行剥除。最后留在叶子节点里面的点即可以为根。
题目：
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.
解法：
因为树就是两个点被一条线链接的无向图，所以先用一个list把树存成无向图的列表。可以从头边同时进行，查看叶子节点并加入到叶子节点链表（遍历一遍后，叶子节点链表size 为1）。 将叶子节点保存下来。这些叶子节点不用再查，但是剩余的中间节点，要依次查看，把跟第一层叶子节点有关的图的矩阵里对应的记录全部删除，类似于剥除最外面一圈所有的点。 这个时候就会有第二层叶子节点（那些在列表当中size为1的点），用同样的方法进行剥除。最后留在叶子节点list里面的点即可以为根。
代码：
