leetcode 669. Trim a Binary Search Tree

class trimBST {
    public TreeNode trimBST(TreeNode root, int low, int high) {
        if(root == null){
            return root;
        }
        if(root.val < low){
            return trimBST(root.right, low, high);
        }
         if(root.val > high){
            return trimBST(root.left, low, high);
        }
        root.left = trimBST(root.left, low, high);

        root.right = trimBST(root.right, low, high);
        return root;
    }
    
  
}