Sudoku solver

题目： 

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.

思路： 回溯法，当有可行解时返回

代码：

class Solution {
    bool isValid(vector<vector<char> >& board, int x, int y)
    {
        bool flags[9] = {0};
        for(int i = 0; i < 9; ++i)
            if(board[x][i] >= '1' && board[x][i] <= '9')
            {
                if(!flags[board[x][i] - '1'])
                    flags[board[x][i] - '1'] = true;
                else
                    return false;
            }
        memset(flags, 0, 9);
        for(int i = 0; i < 9; ++i)
            if(board[i][y] >= '1' && board[i][y] <= '9')
            {
                if(!flags[board[i][y] - '1'])
                    flags[board[i][y] - '1'] = true;
                else
                    return false;
            }
        int xx = x/3*3;
        int yy = y/3*3;
        memset(flags, 0, 9);
        for(int i = 0; i < 3; ++i)
            for(int j = 0; j < 3; ++j)
                if(board[xx+i][yy+j] >= '1' && board[xx+i][yy+j] <= '9')
                {
                    if(!flags[board[xx+i][yy+j]-'1'])
                        flags[board[xx+i][yy+j]-'1'] = true;
                    else
                        return false;
                }
        return true;
    }
public:
    bool solveSudoku(vector<vector<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] = '0' + k;
                        if(isValid(board, i, j) && solveSudoku(board))
                            return true;
                        board[i][j] = '.';
                    }
                    return false;
                }
            }
        return true;
    }
};