class Solution {
public:
    bool isValidBST(TreeNode* root) {
        return CheckIsValidBST(root, numeric_limits<long long>::min(), numeric_limits<long long>::max());
    }

private:
    // [min_val, max_val] - valid subtree range
    bool CheckIsValidBST(TreeNode* node, long long min_val, long long max_val) {
        if (!node) return true;

        if (node->val <= min_val || node->val >= max_val) return false;

        return CheckIsValidBST(node->left, min_val, node->val) && CheckIsValidBST(node->right, node->val, max_val);
    }
};