Connecting dots horizontally and/or vertically as a wall to form a single closed wall.
There should be as many walls on the sides as the number in the cells.
-- 0 : There can be no walls around.
-- The four sides of the cell with 1,2,3 numbers will have as many walls as the number.
-- There is no wall restriction for the edges of the cell where there is no number, there may be no wall at all, or there may be a wall on at most 3 sides.