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By Henry Kwok

Rules of Colour Islands Sudoku

As in a standard Sudoku, the object of the puzzle is to fill in the whole 9x9 grid with numbers 1 through 9 so that each row, each column, and each of the nine 3x3 squares must contain all the nine different numbers 1 through 9.

There are two coloured "islands" in the 9x9 grid. Within each "island" there are two extra rules to apply:
  • no two diagonally adjacent cells can contain the same number
  • EVERY 3x3 square within the islands must contain the numbers 1 through 9.
For example the cell (5,3) contains the number 5 so the cell (6,4) in the yellow island cannot contain a 5 because it is diagonally adjacent to (6,4) - being to the right and down one.
The 9x9 grid with corners at C4R3, C4R5, C6R5 and C6R3 must contain all the digits 1 to 9.