Havel

代码说明：

  这是图论里面的一个定理，即Havel定理，它是用来判断一系列的度的序列能否组成一个简单图，在这儿我实现了它。(This is the inside of a theorem of graph theory, that Havel theorem, which is used to determine whether a series of sequences degrees to form a simple graph, here I realized it.)