class Solution {public:    vector<unordered_set<char>> rows;    vector<unordered_set<char>> cols;    vector<vector<unordered_set<char>>> subboxHm;    bool isValid(char c, int i, int j)    {        return (rows[j].find(c) == rows[j].end() &&                cols[i].find(c) == cols[i].end() &&                subboxHm[j/3][i/3].find(c) == subboxHm[j/3][i/3].end());    }    void putRecord(char c, int i, int j, vector<vector<char> > &board)    {        board[j][i] = c;        rows[j].insert(c);        cols[i].insert(c);        subboxHm[j/3][i/3].insert(c);    }    void clearRecord(char c, int i, int j, vector<vector<char> > &board)    {        board[j][i] = ‘.‘;        rows[j].erase(c);        cols[i].erase(c);        subboxHm[j/3][i/3].erase(c);    }    bool goDFS(vector<vector<char> > &board)    {        size_t cnt = board.size();        for(int j = 0; j < cnt; j ++)        for(int i = 0; i < cnt; i ++)        {            char c = board[j][i];            if (c == ‘.‘)            {                for(char c = ‘1‘; c <= ‘9‘; c ++)                {                    if(isValid(c, i, j))                    {                                                putRecord(c, i, j, board);                        if(goDFS(board))    return true;                        else                clearRecord(c, i, j, board);                    }                }                return false;            }        }        return true;    }    void solveSudoku(vector<vector<char> > &board) {        size_t cnt = board.size();        //    Init        rows.resize(cnt);        cols.resize(cnt);        subboxHm.resize(cnt/3);        for(int i = 0; i < cnt/3; i ++)            subboxHm[i].resize(cnt/3);                //    Fill in original record        for(int j = 0; j < cnt; j ++)        for(int i = 0; i < cnt; i ++)        {            char c = board[j][i];            if (c != ‘.‘)            {                rows[j].insert(c);                cols[i].insert(c);                unordered_set<char> &sb = subboxHm[j/3][i/3];                sb.insert(c);            }        }        goDFS(board);    }};