The square below the X either represents a "1" or the upper part of the first "2". There's absolutely no way to make that square being the lower part of the first "2", because there's no space for that. That way, we can be absolutely sure that there's no block where I put that first X. I use the same reasoning in several other parts of this video.
How did you determine that you could place an X for the first X that you placed? In coordinate (4,4) The column of 1 1 2 2 1 doesn't seem like it gives enough information to determine that a solid block doesn't go there. o_O
The square below the X either represents a "1" or the upper part of the first "2". There's absolutely no way to make that square being the lower part of the first "2", because there's no space for that. That way, we can be absolutely sure that there's no block where I put that first X. I use the same reasoning in several other parts of this video.
Err, I suppose it would be (4,12) from the usual Cartesian plane. My bad. It's still the first X you place!
How did you determine that you could place an X for the first X that you placed? In coordinate (4,4) The column of 1 1 2 2 1 doesn't seem like it gives enough information to determine that a solid block doesn't go there. o_O