LeetCode Day30 valid-sudoku

class Solution {
public:
    bool isValidSudoku(vector<vector<char>>& board) {
        if(board.empty() || board[0].empty()) return false;
        int m=board.size(),n=board[0].size();
        vector<vector<bool>> rowflag(m,vector<bool>(n,false));
        vector<vector<bool>> colflag(n,vector<bool>(m,false));
        vector<vector<bool>> cellflag(m,vector<bool>(n,false));
        for(int i=0;i<m;i++)
            for(int j=0;j<n;j++){
                if(board[i][j]>='0' && board[i][j]<='9'){
                    int c=board[i][j]-'1';
                    if(rowflag[i][c]||colflag[j][c]||cellflag[i/3+(j/3)*3][c]) return false;
                    rowflag[i][c]=true;colflag[j][c]=true;cellflag[i/3+(j/3)*3][c]=true;
                }
            }
        return true;
    }
};