Sudoku Solver, 求数独

public void solveSudoku(char[][] board) {
        solve(board);
    }
    
    public boolean solve(char[][] board)
    {
        for(int i = 0; i < 9; i ++)
        {
            for(int j = 0; j < 9; j ++)
            {
                if(board[i][j] == ‘.‘)
                {
                    for(int k = 1; k <= 9; k ++)
                    {
                        board[i][j] = (char) (k+‘0‘);
                        if(isValid(board, i, j) && solve(board))//当前有多个选择要递归回溯
                        {
                            return true;
                        }
                        board[i][j] = ‘.‘;
                    }
                    return false;
                }
            }
        }
        return true;
    }
    
    public boolean isValid(char[][] board, int i ,int j)
    {
        HashSet<Character> set = new HashSet<>();
        for(int k = 0; k < 9; k ++)
        {
            if(set.contains(board[i][k]))
            {
                return false;
            }
            if(board[i][k]!=‘.‘)
            {
                set.add(board[i][k]);
            }
        }
        set.clear();
        for(int k = 0; k < 9; k ++)
        {
            if(set.contains(board[k][j]))
            {
                return false;
            }
            if(board[k][j]!=‘.‘)
            {
                set.add(board[k][j]);
            }
        }
        set.clear();
        for(int m = 0; m < 3; m ++)
        {
            for(int n = 0; n < 3; n ++)
            {
                int x = i/3*3+m;
                int y = j/3*3+n;
                if(set.contains(board[x][y]))
                {
                    return false;
                }
                if(board[x][y]!=‘.‘)
                {
                    set.add(board[x][y]);
                }
            }
        }
        return true;
    }