LeetCode Sudoku Solver

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

Empty cells are indicated by the character '.'.

You may assume that there will be only one unique solution.

A sudoku puzzle...

...and its solution numbers marked in red.

题意：给出一个数独，填充未填的数。

思路：在遍历未填的数时，去掉该行已填的数，去掉该列已填的数，去掉所在小方块中所填的数，剩下的就是可以填取的数，然后就递归

代码如下：

class Solution
{
    private boolean dfs(int x, int y, char[][] board, Set<Integer> set)
    {
        int row = board.length;
        int col = board[0].length;

        if (y >= col)
        {
            x++; y = 0;
        }

        if (x >= row) return true;

        if (board[x][y] != '.')
        {
            y++;
            if (y >= col)
            {
                x++;y = 0;
            }

            return dfs(x, y, board, set);
        }
        else
        {
            Set<Integer> hs = new HashSet<Integer>(set);
            for (int i = 0; i < col; i++)
            {
                if (board[x][i] != '.')
                {
                    hs.remove(board[x][i] - '0');
                }
            }

            for (int i = 0; i < row; i++)
            {
                if (board[i][y] != '.')
                {
                    hs.remove(board[i][y] - '0');
                }
            }

            int m = (int)Math.sqrt(row);

            int a = x / m, b = y / m;
            for (int i = a * m; i < (a + 1) * m; i++)
            {
                for (int j = b * m; j < (b + 1) * m; j++)
                {
                    if (board[i][j] != '.')
                    {
                        hs.remove(board[i][j] - '0');
                    }
                }
            }

            if (0 == hs.size()) return false;

            for (int num : hs)
            {
                board[x][y] = Character.forDigit(num, 10);
                if (dfs(x, y + 1, board, set)) return true;
                board[x][y] = '.';
            }

            return false;
        }
    }

    public void solveSudoku(char[][] board)
    {
        int row = board.length;
        Set<Integer> hs = new HashSet<Integer>();
        for (int i = 1; i <= row; i++)
        {
            hs.add(i);
        }

        dfs(0, 0, board, hs);

    }
}