%--------------------------
 % @Author: Jingqiao Hu
 % @Date: 2021-02-28 16:31:40
 % @LastEditTime: 2021-02-28 16:32:01
%--------------------------
function bdofs = nodes2dofs(boundarys)
    bdofs = [3*boundarys(:)-2, 3*boundarys(:)-1, 3*boundarys(:)]';
    bdofs = bdofs(:);
end