Sudoku Solver

 1 public class Solution {
 2     public void solveSudoku(char[][] board) {
 3         // IMPORTANT: Please reset any member data you declared, as
 4         // the same Solution instance will be reused for each test case.
 5         solveSudokuRecursive(board);
 6     }
 7     private boolean solveSudokuRecursive(char[][] board) {
 8         int[] pairs = getFirstEmpty(board);
 9         if (pairs[0] == -1 && pairs[1] == -1)
10             return true;
11         for (int i = 1; i <= 9; i++) {
12             board[pairs[0]][pairs[1]] = (char) (i + '0');
13             if (isValid(board, pairs[0], pairs[1])
14                     && solveSudokuRecursive(board)) {
15                 return true;
16             }
17             board[pairs[0]][pairs[1]] = '.';
18         }
19         return false;
20     }
21 
22     private int[] getFirstEmpty(char[][] board) {
23         int[] pairs = null;
24         for (int i = 0; i < 9; i++) {
25             for (int j = 0; j < 9; j++) {
26                 if (board[i][j] == '.') {
27                     pairs = new int[] { i, j };
28                     return pairs;
29                 }
30             }
31         }
32         pairs = new int[] { -1, -1 };
33         return pairs;
34     }
35 
36 
37     private boolean isValid(char[][] board, int x, int y) {
38 
39         for (int i = 0; i < 9; i++) {
40             if (i != y && board[x][i] == board[x][y])
41                 return false;
42 
43             if (i != x && board[i][y] == board[x][y])
44                 return false;
45         }
46 
47         int xIdx = (x / 3) * 3;
48         int yIdx = (y / 3) * 3;
49         for (int m = 0; m < 3; m++) {
50             for (int n = 0; n < 3; n++) {
51                 if(m + xIdx != x && n + yIdx != y && board[m + xIdx][n + yIdx] == board[x][y])
52                     return false;
53             }
54         }
55         return true;
56     }
57 }