Problem: Sudoku Solver

Write a program to solve a Sudoku puzzle by filling the empty cells.

A sudoku solution must satisfy all of the following rules:

Each of the digits 1-9 must occur exactly once in each row. Each of the digits 1-9 must occur exactly once in each column. Each of the digits 1-9 must occur exactly once in each of the 9 3x3 sub-boxes of the grid.

The '.' character indicates empty cells.

class Solution {
   boolean done = false;

    void solve(char[][] board, int row, int col) {
        if (col == board[0].length) {
            row = row + 1;
            col = 0;
        }
         if (row == board.length) {
            done = true;
            return;
        }
        if (board[row][col] == '.') {
            for (int i = 1; i <= 9; i++) {
                if (isSafe(row, col, (char) (i + '0'), board)) {
                    board[row][col] = (char) (i + '0');
                    solve(board, row, col + 1);
                    if (!done)
                        board[row][col] = '.';
                }
            }
        } else
            solve(board, row, col + 1);
    }

    private boolean isSafe(int row, int col, char val, char[][] board) {
        for (int i = 0; i < board.length; i++) {
            if (board[i][col] == val)
                return false;
        }
        for (int i = 0; i < board[0].length; i++) {
            if (board[row][i] == val)
                return false;
        }
        int sR = (row / 3) * 3;
        int eR = sR + 3;
        int sC = (col / 3) * 3;
        int eC = sC + 3;
        for (int i = sR; i < eR; i++) {
            for (int j = sC; j < eC; j++) {
                if (board[i][j] == val)
                    return false;
            }
        }
        return true;
    }

    public void solveSudoku(char[][] board) {
        solve(board, 0, 0);
    }
}