資訊專欄INFORMATION COLUMN
摘要：可以從頭邊同時進行，查看葉子節點并加入到葉子節點鏈表遍歷一遍后，葉子節點鏈表為。將葉子節點保存下來。這個時候就會有第二層葉子節點那些在列表當中為的點，用同樣的方法進行剝除。最后留在葉子節點里面的點即可以為根。
題目：
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：
因為樹就是兩個點被一條線鏈接的無向圖，所以先用一個list把樹存成無向圖的列表。可以從頭邊同時進行，查看葉子節點并加入到葉子節點鏈表（遍歷一遍后，葉子節點鏈表size 為1）。 將葉子節點保存下來。這些葉子節點不用再查，但是剩余的中間節點，要依次查看，把跟第一層葉子節點有關的圖的矩陣里對應的記錄全部刪除，類似于剝除最外面一圈所有的點。 這個時候就會有第二層葉子節點（那些在列表當中size為1的點），用同樣的方法進行剝除。最后留在葉子節點list里面的點即可以為根。
代碼：
