类似symmetric tree判断，只有满足两个数都不是NULL，而且保存值相同，L=L R=R

bool isSameTree(struct TreeNode* p, struct TreeNode* q) {

    if(p == NULL && q == NULL)
        return true;
    else if(p != NULL && q == NULL)
        return false;
    else if(p == NULL && q != NULL)
        return false;
    else{
        if(p->val == q->val)
            return isSameTree(p->left,q->left)&&isSameTree(p->right, q->right);
        else
            return false;
    }
}