36. Valid Sudoku

题目摘要 检查一个数独矩阵是否合法

解法 先检查行，再检查列，再检查每个三行三列的小矩阵

注意 检查三行三列小矩阵时，行标为i + k / 3，列标为j + k % 3

原题 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.