2022-05-08 14:27:21 +0000 UTC

public class NestedIterator implements Iterator<Integer> {

    private List<Integer> integerList = new ArrayList<>();
    private int index = 0;
    public NestedIterator(List<NestedInteger> nestedList) {
        for (NestedInteger nestedInteger : nestedList) {
            flatten(nestedInteger);
        }
    }
    
    private void flatten(NestedInteger nested) {
        if (nested.isInteger()) 
            integerList.add(nested.getInteger());
        else 
            for (NestedInteger nestedFromList : nested.getList()) {
                flatten(nestedFromList);
        }
    }

    @Override
    public boolean hasNext() {
        return index < integerList.size();
    }

    @Override
    public Integer next() {
        return integerList.get(index++);
    }
}