1. class Solution {
  2.     public boolean isValidSudoku(char[][] board) {
  3.         for(int i=0; i<board.length; i++)
  4.         {
  5.             for(int j=0; j<board[0].length; j++)
  6.             {
  7.                 char ch = board[i][j];
  8.                 if(isValid(ch, i, j, board) == false)
  9.                     return false;
  10.             }
  11.         }
  12.         return true;
  13.     }
  14.  
  15.     public boolean isValid(char ch, int row, int col, char[][] board)
  16.     {
  17.         if(board[row][col] == '.')
  18.             return true;
  19.  
  20.         // traversing over 9*9 matrix
  21.         for(int k=0; k<9; k++)
  22.         {
  23.             // checking in same row
  24.             if(board[row][k] == ch && k!=col) // So, k != col ensures we ignore the cell we are currently trying to fill
  25.                 return false;
  26.  
  27.             // checking in same column
  28.             if(board[k][col] == ch && k!=row) // So, k != row ensures we ignore the cell we are currently trying to fill
  29.                 return false;
  30.  
  31.             // starting row of the subgrid (3*3)
  32.             int n = 3*(row/3) + k/3;
  33.             // starting col of the subgrid (3*3)
  34.             int m = 3*(col/3) + k%3;
  35.  
  36.             // we don’t want to compare the cell with itself.
  37.             // If any other cell in the same 3×3 subgrid has ch, it’s not valid
  38.             if( (n!=row || m!=col) && (board[n][m] == ch))
  39.                 return false;
  40.         }
  41.         return true;
  42.     }
  43. }