Pluzzes - Heavier ball

There are the same 8 balls, except just one ball a little bit heavier than the other 7 balls. And now you only have one scale, how do you use the least time to find the heavier ball?

I believe that most people can come up with at least one method to solve it.

Method 1: compare 3 times, which is eleborated in the following picture:



However, this is not the best method. The best method can find the heavier ball with only two comparsions.

we can separate the 8 balls into 3 groups which contain 3 balls, 3 balls, and 2 balls respectively.



So after comparsion group1 and group2, we can understand the heavier ball in which groups.

Next, in each group, there are only 2 or 3 balls, so we can find the solution with one more comparsion.

For 8 balls, we can find the heavier ball with only 2 comparsions. Is it fun ^_-

Note: for 9 balls, the answer is the same. You can try to think about it by yourself.