public static TreeNode BTToDLL1(TreeNode root){
        if(root == null) return null;
        BTToDLL1(root.left);
        if(prev == null){
            head = root;
        }else{
            prev.right = root;
            root.left = prev;
        }
        prev = root;
        BTToDLL1(root.right);
        return head;
    }