[valid sudoku]

1. Example

row      : there exists only 1-9, no duplicate
column: there exists only 1-9, no duplicate
block   : there exists only 1-9, no duplicate

2. Implementation

// Time:O(3* n^2), Space:O(n)
public boolean isValidSudoku( char[][] board )
{



     // validate the input
     if ( board == null || board.length == 0 || board[0].length == 0 ) 
          return false;



     
     // check row     
     for ( int i = 0 ; i < board.length;i++ )
     {
          boolean[] digit = new boolean[9];
          for ( int j = 0 ; j < board[0].length; j++ )
          {
              // NOTE: EMPTY is allowed
              if ( board[i][j] != '.' ) 
              {   
              if (  digit[(int)(board[i][j] - '0')-1]  ) 
              {
                   return false;
              }
              //else
              //{
                   digit[(int)(board[i][j] - '0')-1] = true;
              //} 
              }
          }
     }





     // check column     
     for ( int j = 0 ; j < board[0].length;j++ )
     {
          boolean[] digit = new boolean[9];
          for ( int i = 0 ; i < board.length; i++ )
          {
              // NOTE: EMPTY is allowed
              if ( board[i][j] != '.' )
              {
              if (  digit[(int)(board[i][j] - '0')-1]  ) 
              {
                   return false;
              }
              //else
              //{
                   digit[(int)(board[i][j] - '0')-1] = true;
              //}
              } 
          }
     }




     // check block
     // 0 - 012   1 - 345  2 - 678
     // 3         4        5
     // 6         7        8
     for ( int k = 0 ; k < 9; k++ ) 
     {
          boolean[] digit = new boolean[9];
          //for (int i = (k%3)*3; i < (k%3)*3+3;i++ )
          for (int j = (k/3)*3; j < (k/3)*3+3; j++ )
          {
               //for (int j = (k/3)*3; j < (k/3)*3+3; j++ )
               for (int i = (k%3)*3; i < (k%3)*3+3;i++ )
               {
                   // NOTE: EMPTY is allowed
                   if ( board[i][j] != '.' )
                   {
                   if (  digit[(int)(board[i][j] - '0')-1]  ) 
                   {
                       return false;
                   }
                   //else
                   //{
                       digit[(int)(board[i][j] - '0')-1] = true;
                   //}
                   }                
               }
          }
     }



    
     return true;



}

3. Similar Ones
Sudoku solver