判断一个二叉树是不是另一个二叉树的子结构

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

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