Check all 2x2 squares
A[i][j]   A[i][j+1]
A[i+1][j] A[i+1][j+1]

Make sure you always can cover this square at least one of the methods
x x
y y

or

x y
x y

Edited by author 10.12.2019 14:13

Edited by author 10.12.2019 14:15