public void flatten(TreeNode root) {
        if(root==null){
            return;
        }
        if(root.left!=null){
            TreeNode leftNode=root.left;
            TreeNode rightNode=root.right;
            root.left=null;
            root.right=leftNode;
            TreeNode tmp=leftNode;//root.left has been null
            while(tmp.right!=null){
                tmp=tmp.right;
            }
            tmp.right=rightNode;
        }
        flatten(root.right);
    }