二叉树的镜像

题目
　　请完成一个函数，输入一个二叉树，该函数输出它的镜像．

//把每个非叶子结点的左右字数交换


struct BinaryTreeNode
{
    int     m_nValue;
    BinaryTreeNode* m_pLeft;
    BinaryTreeNode* m_pRight;
    BinaryTreeNode(int n) : m_nValue(n), m_pLeft(NULL), m_pRight(NULL) {}; 
};


void MirrorRecursively(BinaryTreeNode* pNode)
{
    if (pNode == NULL)
        return;
    if (pNode->m_pLeft != NULL || pNode->m_pRight != NULL)
    {
        BinaryTreeNode* pTmp = pNode->m_pLeft;
        pNode->m_pLeft = pNode->m_pRight;
        pNode->m_pRight = pTmp;
        if (pNode->m_pLeft)
            MirrorRecursively(pNode->m_pLeft);
        if (pNode->m_pRight)
            MirrorRecursively(pNode->m_pRight);
    }
}