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'Rainstorm Sudoku' printed from http://nrich.maths.org/
Invented by Henry Kwok
Rules of Rainstorm Sudoku
The variant is given such a name because of the many diagonal
colour regions slanting in one direction, like heavy
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 boxes must contain all the
nine different numbers 1 through 9.
In addition, each diagonal colour region must be filled with
the same number. Note that regions in the same colour on different
parts of the grid do not necessarily contain the same number as