[Leetcode] Valid Sudoku

Problem

Determine if a Sudoku is valid, i.e. each row, column and 3*3 sub-block can only have one number from 1 to 9. 

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

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

Code