Sudoku Solver

Solution:

class Solution {
    public void solveSudoku(char[][] board) {
        if (board == null || board.length == 0
           || board[0].length == 0) return;
        solve(board);
    }
    
    public boolean solve(char[][] board) {
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board[0].length; j++) {
                if (board[i][j] == '.') {
                    for (char c = '1'; c <= '9'; c++) {
                       if (valid(board, i, j, c)) {
                          board[i][j] = c;
                          if (solve(board)) return true;
                          else  board[i][j] = '.';
                       }
                    }
                    return false;
                }
            }
        }
        
        return true;
    }
    
    private boolean valid(char[][] board, int row, int col,  char c) {
        for (int i = 0; i < 9; i++) {
            if (board[row][i] != '.' && board[row][i] == c) return false;
            if (board[i][col] != '.' && board[i][col] == c) return false;
            if (board[3*(row/3) + i/3][3*(col/3) + i%3] != '.' && 
               board[3*(row/3) + i/3][3*(col/3) + i%3] == c) return false;
        }
        
        return true;
    }
}