递归交换左右树

使用递归的思路：每访问一个结点，就交换其左右树。

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x) :
        val(x), left(NULL), right(NULL) {}

};
void Mirror(TreeNode* pRoot) {
    if (pRoot == NULL)
        return ;
    struct TreeNode* t;
    t = pRoot->left;
    pRoot->left = pRoot->right;
    pRoot->right = t;
    Mirror(pRoot->left);
    Mirror(pRoot->right);
}