How to efficiently create a signal of alternating 0- and 1-intervals with different lenghts?

I need to create an array of alternating intervals containing a given number of 0s and 1s defined by different distributions and I need to do this efficiently, because the output array can be very long (2^20 or more). So far, I have come up with this code (P1 and P2 are the distributions of the numbers of 1s and 0s and t is usually just a 1:1:max vector):

function I = signal(t,P1,P2);
% transforms distributions of on- and off-periods to intensity time-trace

p1=[];
p=find(P1);        %%find non-zero elements
ii=1;
while (ii<=length(p))  %%put all periods in a single array
  j=p(ii);
  p1=horzcat(p1,t(j)*ones(P1(j),1)');
  ii=ii+1;
end;
p=randperm(length(p1));  %%shuffle
p1=p1(p);  
p2=[];
p=find(P2);        %%the same for the second distribution
ii=1;
while (ii<=length(p))
  j=p(ii);
  p2=horzcat(p2,t(j)*ones(P2(j),1)');
  ii=ii+1;
end;
p=randperm(length(p2));
p2=p2(p);

I=[];        %%merge
for ii=1:length(p1)
   I=vertcat(I,ones(p1(ii),1),zeros(p2(ii),1));
end;

It works, but the "merge" part takes very long to execute. Is there a way to do this more efficiently?