Sudoku Masyu - 200 Hard Puzzles 9x9 (Volume 21)

Masyu is a type of logic. The purpose of its creation was to present a puzzle that uses no numbers or letters and yet retains depth and aesthetics. Masyu is played on a rectangular grid of squares, some of which contain circles, each circle is either "white" (empty) or "black" (filled). The goal is to draw a single continuous non-intersecting loop that properly passes through all circled cells. The loop must "enter" each cell it passes through from the center of one of its four sides and "e...