Validate Binary Search Tree [LeetCode]

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node's key.
• The right subtree of a node contains only nodes with keys greater than the node's key.
• Both the left and right subtrees must also be binary search trees.

     bool varifyBST(TreeNode * root, int * out_min, int * out_max) {
         if(root == NULL)
             return true;
         (*out_min) = root->val;
         (*out_max) = root->val;
         if(root->left == NULL && root->right == NULL)
             return true;

         int vmax = root->val;
         int vmin = root->val;
         if(root->left != NULL){
             bool ret = varifyBST(root->left, &vmin, &vmax);
             if(ret == false)
                 return false;
             if(vmax >= root->val)
                 return false;
             (*out_min) = min(vmin, root->val);
         }

         if(root->right != NULL){
             bool ret = varifyBST(root->right, &vmin, &vmax);
             if(ret == false)
                 return false;
             if(vmin <= root->val)
                 return false;
             (*out_max) = max(vmax, root->val);
         }
         return true;
     }

     bool isValidBST(TreeNode *root) {
         int vmax = ;
         int vmin = ;
         return varifyBST(root, &vmin, &vmax);
     }