Each islander is given a position to sit on the seesaw for each round. L: sit on the left, R: sit on the right, -: don’t sit on the seesaw.

The pattern for all islanders is below:

person: A B C D E F G H I J K L round 1: L L L L R R R R — — — -

round 2: L L R R R — — — L R L -

round 3: L R R — — L R — L L — R

For example person F will sit on the right, then stand out, then on the left.

After each round, we can see if the seesaw tilted down on the left (L), right (R) or was balanced (-)

The pattern of the seesaw will match the pattern of one person ( or be exactly reversed ). That person is the heavier (or lighter)

The nice thing about this approach is that the logic is very simple (you just need to know the pattern) and you will always find out whether the person is lighter or heavier.

The tricky bit for me was figuring out that the left/right/balance needed to be cross-referenced, then discovering a pattern for each islander, where;