function T = randdtree(n)
% for directred trees
T = zeros(n);
T(1,2) = 1;
for i=3:n
  % attach each node to a random one in the tree
  curnodes = find(sum(T)); curnodes(end+1)=1;
  jj = randperm(i-1); 
  %j = jj(1);
  try
    j = curnodes(jj(1));
  catch
    keyboard
  end
  T(j,i) = 1;
end

% order tree properly
sig = lexordtree(T);
sig = invperm(sig);
T1 = T*0;

[nodesi,nodesj]=find(T);
ne = length(nodesi);
for e=1:ne
  i = nodesi(e);
  j = nodesj(e);
  T1(sig(i),sig(j)) = 1;
end
T = T1;
return