for Robot Artificial Inteligence

11. Valid sudoku

30 Jun 2020 | Daily Algorithms

Three flags are used to check whether a number appear.

used1: check each row

used2: check each column

used3: check each sub-boxes

class Solution
{
public:
    bool isValidSudoku(vector<vector<char> > &board)
    {
        int used1[9][9] = {0}, used2[9][9] = {0}, used3[9][9] = {0};

        for(int i = 0; i < board.size(); ++ i)
            for(int j = 0; j < board[i].size(); ++ j)
                if(board[i][j] != '.')
                {
                    int num = board[i][j] - '0' - 1, k = (i/3) * 3 + j / 3; // It is (i/3)3 not i/(33)
                    if(used1[i][num] || used2[j][num] || used3[k][num])
                        return false;
                    used1[i][num] = used2[j][num] = used3[k][num] = 1;
                }

        return true;
    }
};

This is what k looks like:

0 1 2

3 4 5

6 7 8

Each number represents a 3 * 3 sub-matrix

  • visualization of used3 of k
0  0  0 | 1  1  1 | 2  2  2
0  0  0 | 1  1  1 | 2  2  2
0  0  0 | 1  1  1 | 2  2  2
--------+---------+---------
3  3  3 | 4  4  4 | 5  5  5
3  3  3 | 4  4  4 | 5  5  5
3  3  3 | 4  4  4 | 5  5  5
--------+----------+--------
6  6  6 | 7  7  7 | 8  8  8
6  6  6 | 7  7  7 | 8  8  8
6  6  6 | 7  7  7 | 8  8  8

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