If Max of left Subtree < root && Min of right Subtree > root

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class ResultType {
public:
    TreeNode* minNode;
    TreeNode* maxNode;
    bool isValid;
    ResultType() {
        minNode = NULL;
        maxNode = NULL;
        isValid = true;
    }
};

class Solution {
public:
    /*
     * @param root: The root of binary tree.
     * @return: True if the binary tree is BST, or false
     */
    bool isValidBST(TreeNode * root) {
        // write your code here
        return check(root).isValid;
    }
    ResultType check (TreeNode* root) {
        ResultType result;
        if (root == NULL) {
            return result; 
        }
        ResultType leftNode = check(root->left);
        ResultType rightNode = check(root->right);

        if (!leftNode.isValid || !rightNode.isValid) {
            result.isValid = false;
            return result;
        }
        if (leftNode.maxNode != NULL && leftNode.maxNode->val >= root->val ) {
            result.isValid = false;
            return result;
        }
        if (rightNode.minNode != NULL && rightNode.minNode->val <= root->val) {
            result.isValid = false;
            return result;
        }
        if (leftNode.minNode != NULL) {
            result.minNode = leftNode.minNode;
        } else {
            result.minNode = root;
        }
        if (rightNode.maxNode != NULL) {
            result.maxNode = rightNode.maxNode;
        } else {
            result.maxNode = root;
        }
        return result;
    }

};