分析器的工作是分离一串文本并返回一个标号流。

The analyzer's job is to take apart a string of text and give you back a stream of tokens.

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给定一个无向图，编写函数找出所有的最小高度树，并返回其根标号的列表。

Given such a graph, write a function to find all the MHTs and return a list of their root labels.

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