Hard Sudoku

All Sudoku have a solution and this solution is unique. The game has four levels of difficulty. To solve Sudoku, use the following strategies: LEVEL 1: "naked pair", "hidden pair", "box-line", "pointing pair", "pointing triple" LEVEL 2: "naked triple", "naked quad", "hidden triple", "Y-wing", "X-wing" LEVEL 3: "XY-chain", "rectangle", "swordfish", "jellyfish", "hidden quad" LEVEL 4: "trial and error"