function a = zigzag3(n)
  a = zeros(n, n);
  for k=1:n
    d = (2*(j = mod(k, 2))-1)*(n-1);
    m = (n-1)*(k-1);
    a(k+(1-j)*m:d:k+j*m) = k*(k-1)/2:k*(k+1)/2-1;
    a(n*(n+1-k)+(1-j)*m:d:n*(n+1-k)+j*m) = n*n-k*(k+1)/2:n*n-k*(k-1)/2-1;
  endfor
endfunction

>> zigzag3(5)
ans =

    0    1    5    6   14
    2    4    7   13   15
    3    8   12   16   21
    9   11   17   20   22
   10   18   19   23   24