[LeetCode] Valid Sudoku

Problem:

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.

Solution:O(n^2)

public class Solution {
   public boolean isValidSudoku(char[][] board){
      boolean [][] rows=new boolean[9][9];

      boolean [][] cols=new boolean[9][9];

      boolean [][] blocks=new boolean[9][9];

      for (int i = 0; i < 9; i++) {  

         for (int j = 0; j < 9; j++) {

            int c = board[i][j] - '1';
            if (board[i][j] == '.') 
               continue;  

            int index = i - i % 3+ j / 3; 

            if (rows[i][c] || cols[j][c] || blocks[index][c])  

               return false;  
            rows[i][c] = cols[j][c] = blocks[index][c] = true;  

         }  
      }  
      return true;  
   }
}

Key Concept: use i,j to get it's block index