251. Flatten 2D Vector

class Vector2D {
public:
    Vector2D(vector<vector<int>>& vec2d) {
        rowBegin = vec2d.begin();
        rowEnd = vec2d.end();
        if (rowBegin != rowEnd) colBegin = rowBegin->begin();
    }

    int next() {
        hasNext();
        return *(colBegin++);
    }

    bool hasNext() {
        while (rowBegin != rowEnd && colBegin == rowBegin->end()) {
            rowBegin++;
            colBegin = rowBegin->begin();
        }
        return rowBegin != rowEnd;
    }
private:
    vector<vector<int>>::iterator rowBegin, rowEnd;
    vector<int>::iterator colBegin;
};

/**
 * Your Vector2D object will be instantiated and called as such:
 * Vector2D i(vec2d);
 * while (i.hasNext()) cout << i.next();
 */