37. Sudoku Solver

    /*
     * 37. Sudoku Solver
     * 2015.12.13 by Mingyang
     * 1.长度标准：无
     * 2.可选的范围：所有木有值得点，选取从1到9的数字
     * 3.往前走一步：如果放进去是validate的，那就放
     * 4.后退一步：若果放进去，后面是false的，就把这个点改回来
     * 5.特别的case：无了
     * 6.关于重复：无
     * 本题目的isValid是难点
     * row = i / 3 * 3; row < i / 3 * 3 + 3
     */
    public void solveSudoku(char[][] board) {
        if (board == null || board.length == 0)
            return;
        helper(board);
    }
    private boolean helper(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 num = '1'; num <= '9'; num++) {// 尝试
                        if (isValid(board, i, j, num)) {
                            board[i][j] = num;
                        if (helper(board))//这个点暂时合适，那么进入下一层
                                return true;
                            else
                                board[i][j] = '.';// 回退
                        }
                    }
                    return false;
                }
            }
        }
        return true;
    }
    private boolean isValid(char[][] board, int i, int j, char c) {
        // check column
        for (int row = 0; row < 9; row++)
            if (board[row][j] == c)
                return false;
        // check row
        for (int col = 0; col < 9; col++)
            if (board[i][col] == c)
                return false;
        // check block
        for (int row = i / 3 * 3; row < i / 3 * 3 + 3; row++)
            for (int col = j / 3 * 3; col < j / 3 * 3 + 3; col++)
                if (board[row][col] == c)
                    return false;
        return true;
    }