I saw the headings of this video and thought, oh come on, why in the world is this considered difficult? There are so many numbers already given!
So, quickly went to the site and started.
Only a few minutes after starting did I realizing the order and patterns in the puzzle: do you see it? The diagonal 1-2-3 and the 4-5-6 and the 7-8-9. I only saw it after getting stuck in the middle.
The reason it's diabolical is because on the bottom 3 rows (where is there no immediately discernible pattern) on the left side, it is impossible to immediately (or even after some head-scratching) figure out where the 4,5,6 go. And on the right side of those 3 rows, it is impossible to figure out where the 1,2, and 3 go.
Ok.
Guess this needs a lot more investment of mental energy than I assumed.
