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.

数独的规则很简单：

1. 横着的9个数，不能重复
2. 竖着的9个数，不能重复
3. 3×3小方块中的9个数，也不能重复

那么解题思路，就是依次检查这三个方面有没有重复。