Huffman树 建树方法代码实现

照着书上给的代码段改了一上午BUG，最后发现是书上代码有问题。。。

#include<iostream>
#include<queue>
using namespace std;
template <class T> class Htree;
template <class T>
class Node{ //结点类
    friend class Htree<T>;
public:
    T data;
    Node *par;
    Node *left;
    Node *right;
    Node()
    {
        par=left=right=NULL;
    }
    Node(const T value,Node<T> *l,Node<T> *r)
    {
        data=value;
        left=l;
        right=r;
    }
    Node<T>& operator =(const Node &n)
    {
        data=n.data;
        par=n.par;
        left=n.left;
        right=n.right;
    }
    friend const bool operator<(const Node &a,const Node &b) //重载运算符，用于优先队列
    {
        return a.data>b.data;
    }
};
template <class T>
class Htree{
public:
    Node<T> *root;
    Htree(T w[],int n)
    {
        priority_queue<Node<T> > heap; //小根堆，每次取出值最小的两个结点
        Node<T> first,second,*p,*q;
        Node<T> *List=new Node<T>[n];
        for(int i=0;i<n;i++)
        {
            List[i].data=w[i];
            List[i].par=List[i].right=List[i].left=NULL;
            heap.push(List[i]);
        }
        for(int i=0;i<n-1;i++)
        {
            first=(heap.top());
            heap.pop();
            second=(heap.top());
            heap.pop();
            p=new Node<T>(first.data,first.left,first.right); //取出后一定要新建结点，不然会把树的指针变成环
            q=new Node<T>(second.data,second.left,second.right); //同上
            Node<T> *parent=new Node<T>(first.data+second.data,p,q); //新建一个父节点，并将前两个结点设为子树
            heap.push(*parent);
            root=parent;
        }
        delete [] List;
    }
    void PreOrder(Node<T> *r) //先序遍历
    {
        if(r==NULL)return;
        cout << r->data << " ";
        PreOrder(r->left);
        PreOrder(r->right);
    }
    void InOrder(Node<T> *r) //中序遍历
    {
        if(r==NULL)return;
        InOrder(r->left);
        cout << r->data <<" ";
        InOrder(r->right);
    }
};

int main()
{
    int n;
    int a[1000],s;
    while(cin>>n)
    {
        for(int i=0;i<n;i++)
        {
            cin>>a[i];
        }
        Htree<int> t(a,n);
        cout <<"preorder:"<<endl;
        t.PreOrder(t.root);
        cout <<endl;
        cout <<"inorder:"<<endl;
        t.InOrder(t.root);
        cout <<endl;
    }
}