From Wikipedia - The n-puzzle is known in various versions, including the 8 puzzle, the 15 puzzle, and with various names (Gem Puzzle, Boss Puzzle, Game of Fifteen, Mystic Square and many others). It is a sliding puzzle that consists of a frame of numbered square tiles in random order with one tile missing. If the size is 3×3, the puzzle is called the 8-puzzle or 9-puzzle, and if 4×4, the puzzle is called the 15-puzzle or 16-puzzle. The object of the puzzle is to place the tiles in order (see diagram) by making sliding moves that use the empty space.
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