题目：
Implement an iterator to flatten a 2d vector.
For example, Given 2d vector =
[ [1,2], [3], [4,5,6] ]
By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,2,3,4,5,6].
Follow up: As an added challenge, try to code it using only iterators in C++ or iterators in Java.
思路：
需要维护两个动态迭代器row_it和col_it，分别表示当前位置的行和列。但是由于我们不存储vec2d本身，所以我们还需要另外一个静态迭代器row_end，表示整个二维数组的末尾。这里需要特别注意的是在判断hasNext的时候，要处理那种跳过空行的情况，请见下面的代码实现。
代码：
class Vector2D { public: Vector2D(vector<vector<int>>& vec2d) { if (vec2d.size() == 0) { return; } row_it = vec2d.begin(), row_end = vec2d.end(); col_it = (*row_it).begin(); } int next() { return *col_it++; } bool hasNext() { while (row_it != row_end && col_it == (*row_it).end()) { ++row_it; col_it = (*row_it).begin(); } return row_it != row_end; } private: vector<vector<int>>::iterator row_it, row_end; vector<int>::iterator col_it; }; /** * Your Vector2D object will be instantiated and called as such: * Vector2D i(vec2d); * while (i.hasNext()) cout << i.next(); */