code.java

public void interleave(ArrayList<Integer> a1, ArrayList<Integer> a2){
    Iterator<Integer> it2 = a2.iterator();
    int idx = 0;
    while (it2.hasNext()){
        idx++;
        if (idx < a1.size()){
            a1.add(idx, it2.next());
        } else {
            a1.add(it2.next());
        }
    }
}