Interview questions 04.05. Legitimate binary search tree

This is a divide and conquer find the minimum + maximum. Binary search tree root node than the left subtree maximum but smaller than a minimum right subtree.

class Solution {

private:

public:

    bool isValidBST(TreeNode* root) {

        if(root == NULL) return true;

        TreeNode* max_left = root->left;

        TreeNode* min_right = root->right;

        while(max_left != NULL && max_left->right != NULL){

            max_left = max_left->right;

        }

        while(min_right != NULL && min_right->left != NULL){

            min_right = min_right->left;

        }

        bool result = true;

        if(max_left) result = result && root->val > max_left->val;

        if(min_right) result = result && root->val < min_right->val;

        return isValidBST(root->left) && isValidBST(root->right) && result;

    }

};