valid sudoku

Determine if a Sudoku is valid, according to: Sudoku Puzzles - The Rules.

The Sudoku board could be partially filled, where empty cells are filled with the character '.'.

A partially filled sudoku which is valid.

Note:
A valid Sudoku board (partially filled) is not necessarily solvable. Only the filled cells need to be validated.

There are just 3 rules to Sudoku.
Each row must have the numbers 1-9 occuring just once.
Each column must have the numbers 1-9 occuring just once.
And the numbers 1-9 must occur just once in each of the 9 sub-boxes of the grid.

reference:http://www.cnblogs.com/springfor/p/3884217.html

先按每行check，如果是'.'说明还没填字，是合法的，往下走，如果没在set中存过就加一下，如果便利过程中出现了在set中存在的key值，说明有重复的数字在一行，不合法，return false。

再按照这个方法check列。

最后按照这个方法check小方块。

注意小方块的ij取法。对于当前这块板子来说，总共有9个小方格，按0~8从左到右依次编号。

按编号求'/'就是求得当前小方格的第一行横坐标，因为每个小方格有3行，所以循环3次。

按编号求'%'就是求得当前小方格的第一列纵坐标，因为每个小方格有3列，所以循环3次。 

对9个小方格依次走一边，就完成了检查小方格的工作。