public boolean isValidBST(TreeNode root) {

if (root == null) return true;

return dfs(root, null, null);

}

private boolean dfs(TreeNode root, Integer min, Integer max) {

if (root == null) return true;

if (

(min != null && root.val <= min) || max != null && root.val >= max

) {

return false;

}

return dfs(root.left, min, root.val) && dfs(root.right, root.val, max);

}