LeetCode - Sudoku Solver

package sudoku;

public class SudokuSolver {

    public void solveSudoku(char[][] board) {
        solve(board);
    }
    
    private boolean solve(char[][] board) {
        int m = -1;
        int n = -1;
        for (int i = 0; i < 9; ++i) {
            for (int j = 0; j < 9; ++j) {
                if (board[i][j] == ‘.‘) {
                    m = i;
                    n = j;
                    break;
                }
            }
        }
        
        if (m == -1 && n == -1) {
            return true;
        }
        
        for (int k = 1; k <= 9; ++k) {
            board[m][n] = (char)(‘0‘ + k);
            if (validate(board, m, n) && solve(board))
                return true;
            board[m][n] = ‘.‘;
        }
        
        return false;
    }
    
    private boolean validate(char[][] board, int x, int y) {
        for (int i = 0; i < 9; ++i) {
            if (i != y &&board[x][i] == board[x][y])
                return false;
            
            if (i != x && board[i][y] == board[x][y]) 
                return false;
        }
        
        int m = x / 3;
        int n = y / 3;
        for (int i = 3*m; i < 3*m + 3; ++i) {
            for (int j = 3*n; j < 3*n + 3; ++j) {
                if (i != x && j != y && board[x][y] == board[i][j])
                    return false;
            }
        }
        
        return true;
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        char[][] board = {
                { ‘5‘, ‘3‘, ‘.‘, ‘.‘, ‘7‘, ‘.‘, ‘.‘, ‘.‘, ‘.‘ },
                { ‘6‘, ‘.‘, ‘.‘, ‘1‘, ‘9‘, ‘5‘, ‘.‘, ‘.‘, ‘.‘ },
                { ‘.‘, ‘9‘, ‘8‘, ‘.‘, ‘.‘, ‘.‘, ‘.‘, ‘6‘, ‘.‘ },
                { ‘8‘, ‘.‘, ‘.‘, ‘.‘, ‘6‘, ‘.‘, ‘.‘, ‘.‘, ‘3‘ },
                { ‘4‘, ‘.‘, ‘.‘, ‘8‘, ‘.‘, ‘3‘, ‘.‘, ‘.‘, ‘1‘ },
                { ‘7‘, ‘.‘, ‘.‘, ‘.‘, ‘2‘, ‘.‘, ‘.‘, ‘.‘, ‘6‘ },
                { ‘.‘, ‘6‘, ‘.‘, ‘.‘, ‘.‘, ‘.‘, ‘2‘, ‘8‘, ‘.‘ },
                { ‘.‘, ‘.‘, ‘.‘, ‘4‘, ‘1‘, ‘9‘, ‘.‘, ‘.‘, ‘5‘ },
                { ‘.‘, ‘.‘, ‘.‘, ‘.‘, ‘8‘, ‘.‘, ‘.‘, ‘7‘, ‘9‘ }
                };
        SudokuSolver s = new SudokuSolver();
        s.solveSudoku(board);
    }

}