There are 9 rows of numbers across and 9 columns of numbers down.
Numbers 1 through 9 need to go across and down.
You can only use each number once in each row, column, or 3×3 square.

l _ l _ l _ ll 6 l 9 l _ ll 8 l _ l _ l
l _ l 6 l 8 ll _ l _ l _ ll 2 l 3 l _ l
l 9 l _ l 2 ll 3 l _ l _ ll _ l _ l _ l

l 4 l _ l _ ll _ l 3 l 1 ll _ l _ l 7 l
l 8 l 2 l _ ll _ l _ l _ ll _ l 1 l 3 l
l 6 l _ l _ ll 7 l 2 l _ ll _ l _ l 5 l

l _ l _ l _ ll _ l _ l 3 ll 1 l _ l 4 l
l _ l 4 l 3 ll _ l _ l _ ll 7 l 5 l _ l
l _ l _ l 9 ll _ l 5 l 4 ll _ l _ l _ l