601. Flatten 2D Vector [LintCode]

Implement an iterator to flatten a 2d vector.

Example

Given 2d vector =

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

public class Vector2D implements Iterator<Integer> {

    public Vector2D(List<List<Integer>> vec2d) {
        // Initialize your data structure here
    }

    @Override
    public Integer next() {
        // Write your code here
    }

    @Override
    public boolean hasNext() {
        // Write your code here
    }

    @Override
    public void remove() {}
}

/**
 * Your Vector2D object will be instantiated and called as such:
 * Vector2D i = new Vector2D(vec2d);
 * while (i.hasNext()) v[f()] = i.next();
 */