Write a program to solve a Sudoku puzzle by filling the empty cells.Empty cells are indicated by the character ‘.‘.You may assume that there will be only one unique solution.

 1 public class Solution { 2     public void solveSudoku(char[][] board) { 3         if (board == null || board.length != 9 || board[0].length != 9) return; 4         helper(board, 0, 0); 5     } 6      7     public boolean helper(char[][] board, int i, int j) { 8         if (j >= 9) return helper(board, i+1, 0); 9         if (i >= 9) return true;10         if (board[i][j] == ‘.‘) {11             for (int k = 1; k <= 9; k++) {12                 board[i][j] = (char)(‘0‘ + k);13                 if (isvalid(board, i, j)) {14                     if(helper(board, i, j+1))15                     return true;16                 }17                 board[i][j] = ‘.‘;18             }19         }20         else {21             return helper(board, i, j+1);22         }23         return false;24     }25     26     public boolean isvalid(char[][] board, int i, int j) {27         for (int a = 0; a < 9; a++) {28             if (a != i && board[a][j] == board[i][j]) return false;29         }30         31         for (int b = 0; b < 9; b++) {32             if (b != j && board[i][b] == board[i][j]) return false; 33         }34         35         for (int c = i/3*3; c < i/3*3 + 3; c++) {36             for (int d = j/3*3; d < j/3*3 + 3; d++) {37                 if ((c != i || d != j) && board[c][d] == board[i][j]) return false;38             }39         }40         41         return true;42     }43 }