Ring-Ring (Masyu) (1054)

Draw multiple loops that pass orthogonally through centers of cells. Each loop must be rectangular in shape (squaresare considered rectangles). Every cell must be used in at least one loop. Loops may intersect, but only in cells without circles, and both loops must be going straight through those cells. The loop must go straight through the cells with white circles, with a turn in at least one of the cells immediately before or after each white circle. The loop must make a turn in all the black circles, but must go straight in both cells immediately before and after each black circle.

Reference: WPF (1)