Sudoku solver using depth-first search

sudoku_solver.java

import java.io.*;
import java.util.*;

class Main {
    public static void main(String[] args) throws IOException {
        File file = new File(args[0]);
        char[][] board = new char[9][9];
        BufferedReader buffer = new BufferedReader(new FileReader(file));
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                board[i][j] = (char)buffer.read();
            }
            buffer.read();
        }
        Solution sol = new Solution();
        sol.solveSudoku(board);
    }
}

class Solution {
    public void solveSudoku(char[][] board) {
        for (int pos = 0; pos < 81; pos++) {
            if (board[pos/9][pos%9] == '.') {
                tryValues(board, pos/9, pos%9);
                break;
            }
        }
    }
    
    void printBoard(char[][] board) {
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                System.out.printf("%c ", board[i][j]);
            }
            System.out.println();
        }
        System.out.println("_________________");
    }
    
    // test for blank space before using this function
    boolean tryValues(char[][] board, int x, int y) {
        // identifying unavailable values
        HashSet<Character> values = new HashSet();
        for (int i = 0; i < 9; i++) {
            if (board[x][i] != '.') {
                values.add(board[x][i]);
            }
        }
        for (int i = 0; i < 9; i++) {
            if (board[i][y] != '.') {
                values.add(board[i][y]);
            }
        }
        int gx = x/3;
        int gy = y/3;
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 3; j++) {
                if (board[3*gx+i][3*gy+j] != '.') {
                    values.add(board[3*gx+i][3*gy+j]);
                }
            }
        }
        if (values.size() == 9) {
            return false;
        }
        // find next empty slot, solved if none left
        int pos = 9*x+y+1;
        for (; pos < 81; pos++) {
            if (board[pos/9][pos%9] == '.') {
                break;
            }
        }
        if (pos == 81) {
            for (int c = 49; c <= 57; c++) {
                if (!values.contains((char)c)) {
                    board[x][y] = (char)c;
                }
            }
            System.out.println("Finished!");
            printBoard(board);
            return true;
        }
        // try available values and backtrack
        boolean success = false;
        for (int c = 49; c <= 57; c++) {
            if (!values.contains((char)c)) {
                board[x][y] = (char)c;
                success = tryValues(board, pos/9, pos%9);
                if (success) {
                    return true;
                }
            }
        }
        board[x][y] = '.';
        return false;
    }
}

zzz.txt

....58..3
......4..
6..2..1..
.3...1.5.
.72.9..8.
..8..5.3.
....1....
35.......
.913....2