Place a horizontal or a vertical line in every blank cell. A number indicates the total length of the segments connected to that square.
There are only three cells visible from the digit 3, so all of these cells need to be connected to it. The 2 now already has one out of two segments assigned to it and the second one can only be placed above it. Because it should not have three segments, the wall in the upper left corner needs to be a horizontal one. The wall in the middle square should also be a horizontal one, otherwise the 1 will have two segments.
This genre was invented by Naoki Inaba.
Puzzles in this genre