LeetCode - Flatten 2D Vector

Flatten2DVector.java

// 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].

public class Vector2D implements Iterator<Integer> {
    List<Iterator> iterators = new ArrayList<Iterator>();
    int index = 0;
    public Vector2D(List<List<Integer>> vec2d) {
        if(vec2d != null && vec2d.size() != 0){
            for(List<Integer> lst : vec2d){
                if(lst != null && lst.size() != 0) iterators.add(lst.iterator());
            }
        }
    }

    @Override
    public Integer next() {
        while(index < iterators.size()){
            if(iterators.get(index).hasNext()) return (Integer)iterators.get(index).next();
            index++;
        }
        return null;
    }

    @Override
    public boolean hasNext() {
        if(index == iterators.size()) return false;
        else if(index < iterators.size() - 1) return true;
        else return iterators.get(index).hasNext();
    }
}

/**
 * Your Vector2D object will be instantiated and called as such:
 * Vector2D i = new Vector2D(vec2d);
 * while (i.hasNext()) v[f()] = i.next();
 */
 
 // https://leetcode.com/discuss/50292/7-9-lines-added-java-and-c-o-1-space