how to decrease time of for loop

if I have data X=[1 3; 1 4; 2 3; 2 6] (where max number in X (6) represent the number of nodes) that represent the edges between nodes of network and i want to calculate the remaining edge =[1 2; 1 5;1 6;2 4;2 5;3 4;3 5;3 6;4 5;4 6;5 6]

thus i calculate the all links(result) then remove X from it.

but when i have max number of nodes (say 8000) , the following code take a long time . So I want another code take a short time.

u=max(max(X)); %u is the number of nodes in graph  
result = zeros(u*2,2);
ri = 1;
for ii=1:u
for jj = ii+1:u
result(ri,:) = [ii jj];
ri = ri+1;
end
end

[muhammad ismat]

isRowToRemove = ismember(result,X,'rows');%test result is member in X or not, it is removed if it is member in X.  
result(isRowToRemove,:) = [];