For a test case:
   if it should be output " a  b ", it also could be output
" b  a ",  which one is correct?
   for a test case,for example:
                4
                1 0
                0 1
                1 1
                2 2
  we could select not only 1 3, but also 3 4,
  which is the correct one ?

I think, you can take any of them