c/c++哈夫曼编码

函数代码:

void HuffTree::genHuffCode()                                // 为哈夫曼树中各叶子的字符生成哈夫曼编码
{
    int start = MAX_CODE_LENGTH - 1;            
    for (int i = 1; i <= n; ++i)
    {
        int c = i;
        int f = tree[c].parent;
        while (f!=0)
        {
            if (tree[f].lchild == c)
            {
                tree[c].code[start] = '0';
            }
            else
            {
                tree[c].code[start] = '1';
            }
            c = f;
            f = tree[c].parent;
            --start;
        }
        for (int j = 0; j < MAX_CODE_LENGTH / 2; j++)
        {
            char t = tree[i].code[j];
            tree[i].code[j] = tree[i].code[MAX_CODE_LENGTH - j - 1];
            tree[i].code[MAX_CODE_LENGTH - j - 1] = t;
        }
    }
}

问题：
在f=tree[c].parent处异常终止
引发了异常: 读取访问权限冲突。
this 是 0x31A49F0A。