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# 251 Flatten 2D Vector

Implement an iterator to flatten a 2d vector.

For example,\
Given 2d vector =

```
[
  [1,2],
  [3],
  [4,5,6]
]
```

By callingnextrepeatedly untilhasNextreturns false, the order of elements returned bynextshould be:`[1,2,3,4,5,6]`.

**Follow up:**\
As an added challenge, try to code it using only [iterators in C++](http://www.cplusplus.com/reference/iterator/iterator/) or [iterators in Java](http://docs.oracle.com/javase/7/docs/api/java/util/Iterator.html).

**The Idea:** Maintain a row and column pointer, and update these for each `next()` call. The row will increments as the column will reset to 0, when the column index is going to exceed it's current row vector. One challenge was dealing with empty vectors `{}`. These need to be ignored and iterated over every time we enter the start of a new row.

**Complexity:** O(n) time where n is the total number of elements in the vector.

```cpp
class Vector2D {
public:
    Vector2D(vector<vector<int>>& vec2d) {
        v = &vec2d;
        r = c = 0;
        _skip_empty_vectors();
    }

    int next() {
        // current r and c should always be valid
        int cur_row = r, cur_col = c;

        // update for next call
        if (cur_col + 1 >= v->at(cur_row).size()) {
            c = 0;
            r++;
            _skip_empty_vectors();
        }
        else c++;

        return v->at(cur_row).at(cur_col);
    }

    bool hasNext() {
        return r < v->size();
    }

private:
    int r, c;
    vector<vector<int>> *v;

    void _skip_empty_vectors() {
        while (hasNext() && v->at(r).empty()) {
            r++;
        }
    }
};
```


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