>> This i don't really understand
This is what I was trying to explain earlier. You cannot add a column to the solution more than once, right? So in your example, in your first step you see that column 1 has the highest value for k. So you add column 1 to the solution. The next time through, you don't want to add column 1 to the solution again, so you should not calculate k for it. It should be ignored in the calculation.