public class NestedIterator implements Iterator<Integer> {
    private NestedInteger val;
    private Stack<Iterator<NestedInteger>> its;

    public NestedIterator(List<NestedInteger> list) {
        its = new Stack<>();

        its.push(list.iterator());

    }

    @Override
    public Integer next() {
        return val.getInteger();
    }

    @Override
    public boolean hasNext() {
        while(!its.isEmpty()) {
            Iterator<NestedInteger> it = its.peek();
            if (!it.hasNext()) its.pop();
            else {
                NestedInteger ni = it.next();
                if (ni.isInteger()) {
                    val = ni;
                    return true;
                } else {
                    its.push(ni.getList().iterator());
                }
            }
        }

        return false;
    }
}