树的子结构

输入两棵二叉树A，B，判断B是不是A的子结构。（ps：我们约定空树不是任意一个树的子结构）

class Solution {public:    bool HasSubtree(TreeNode* pRoot1, TreeNode* pRoot2)    {        if(pRoot2 == NULL || pRoot1 == NULL )            return false;        return isSubtree(pRoot1, pRoot2)|| HasSubtree(pRoot1->left,pRoot2) || HasSubtree(pRoot1->right,pRoot2);    }          bool isSubtree(TreeNode* pRoot1 , TreeNode* pRoot2){        if(pRoot2 == NULL)            return true;        if(pRoot1 == NULL)            return false;        return pRoot1->val == pRoot2->val && isSubtree(pRoot1->left,pRoot2->left) && isSubtree(pRoot1->right,pRoot2->right);    }};