leetcode 98. Validate Binary Search Tree

bool isValidBST(TreeNode *root) 
    {
        return BSTCheck(root, (long long)INT_MIN - 1, (long long)INT_MAX + 1);
    }
    
    bool BSTCheck(TreeNode *root, long long left, long long right)
    {
        if (root == NULL)
            return true;
        return ((long long)root->val > left) && ((long long)root->val < right) 
            && BSTCheck(root->left, left, root->val)
            && BSTCheck(root->right, root->val, right);
    }

 1 bool isValidBST(TreeNode *root) 
 2     {
 3         vector<int> vtree;
 4         
 5         if (root == NULL)
 6             return true;
 7         
 8         inorder(root, vtree);
 9         for (int i = 0; i < vtree.size() - 1; i++)
10         {
11             if (vtree[i] >= vtree[i + 1])
12                 return false;
13         }
14         return true;
15     }
16     
17     void inorder(TreeNode *root, vector <int> &ret)
18     {
19         if (root == NULL)
20             return;
21         inorder(root->left, ret);
22         ret.push_back(root->val);
23         inorder(root->right, ret);
24     }