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

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。

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Players had to map out their progress in virtual worlds on sheets of graph paper, or write down lengthy passwords to save games.

WSJ: These Videogame Sites Are for 'Mature' Audiences Only