Leetcode: Sudoku Solver

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 }