HuffmanTree(E w[], int n)构造函数
HuffmanNode<T, E>* getRoot()获取根节点
void output(HuffmanNode<T, E>* t, string str, ostream& out)在控制台上输出这棵树
void mergeTree(HuffmanNode<T, E>* ht1, HuffmanNode<T, E>* ht2,HuffmanNode<T, E>*& parent)两颗子树合并
HuffmanNode<T, E>* root根节点