filll-updates.cpp

using cellQueue = std::deque<SudokuCell>;

class SudokuBoard {
    /*
     * fill the board with valid solution
     */
    void fillCells() {
        auto remaininCells = cellQueue{cells};

        if(!doFillCells(remaininCells)) {
            cout << "Unable to fill board" << endl;
        }
    }

    bool doFillCells(cellQueue &remainingCells) {
        // get first cell and tail
        auto cell = remainingCells.front();
        remainingCells.pop_front();

        // collected from all the neighbor values
        set<int> neighborValues = {};
        // produced by set difference VALID_VALUES \ neighbord values
        vector<int> options;
        
        for(auto option : options) {
            cell->value = option;

            // base case (no remaining cells)
            if (remainingCells.empty()) {
                return true;
            }
            
            // recursive step
            if (doFillCells(remainingCells)){
                return true;
            };
        }

        // base case (push cell back to set, and backtrack)
        cell->value = 0;
        remainingCells.push_front(cell);
        return false;
    }
}