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"