251. Flatten 2D Vector

O(1) time O(1) space

class Vector2D {
public:
    Vector2D(vector<vector<int>>& v) : i(begin(v)), e(end(v)), j(0) {
        adjust();
    }

    int next() {
        int res = i->at(j++);
        adjust();
        return res;
    }

    bool hasNext() {
        return i != e;
    }

    void adjust() {
        while (i != e && j >= i->size()) {
            ++i;
            j = 0;
        }
    }

    vector<vector<int>>::iterator i, e;
    int j;
};

/**
 * Your Vector2D object will be instantiated and called as such:
 * Vector2D* obj = new Vector2D(v);
 * int param_1 = obj->next();
 * bool param_2 = obj->hasNext();
 */

class Vector2D {
public:
    Vector2D(vector<vector<int>>& v) : i(begin(v)), e(end(v)), j(0) {

    }

    int next() {
        hasNext(); // adjustment
        return i->at(j++);
    }

    bool hasNext() {
        while (i != e && j >= i->size()) {
            ++i;
            j = 0;
        }
        return i != e;
    }

    vector<vector<int>>::iterator i, e;
    int j;
};

/**
 * Your Vector2D object will be instantiated and called as such:
 * Vector2D* obj = new Vector2D(v);
 * int param_1 = obj->next();
 * bool param_2 = obj->hasNext();
 */

class Vector2D {
public:
    Vector2D(vector<vector<int>>& v) : it1(begin(v)), e(end(v)) {
        while (hasNext() && it1->empty()) {
            ++it1;
        }
        it2 = hasNext() ? it1->begin() : it2;
    }

    int next() {
        int res = *it2++;
        while (it2 == it1->end() && ++it1 != e) {
            it2 = it1->begin();
        }
        return res;
    }

    bool hasNext() {
        return it1 != e;
    }

    vector<vector<int>>::iterator it1, e;
    vector<int>::iterator it2;
};

/**
 * Your Vector2D object will be instantiated and called as such:
 * Vector2D* obj = new Vector2D(v);
 * int param_1 = obj->next();
 * bool param_2 = obj->hasNext();
 */