# Flatten Nested List Iterator

#1

Given a nested list of integers, implement an iterator to flatten it.

Each element is either an integer, or a list -- whose elements may also be integers or other lists.

Example 1:

```Input: [[1,1],2,[1,1]]
Output: [1,1,2,1,1]
Explanation: By calling next repeatedly until hasNext returns false,
the order of elements returned by next should be: `[1,1,2,1,1]`.```

Example 2:

```Input: [1,[4,[6]]]
Output: [1,4,6]
Explanation: By calling next repeatedly until hasNext returns false,
the order of elements returned by next should be: `[1,4,6]`.
```

#2

The below code uses a `queue` to keep track of the order.

``````public class NestedIterator implements Iterator<Integer> {
public NestedIterator(List<NestedInteger> nestedList) {
helper(nestedList);
}

private void helper(List<NestedInteger> list){
if(list==null) return;
for(NestedInteger ni : list){
if(ni.isInteger()){
queue.offer(ni.getInteger());
} else{
helper(ni.getList());
}
}
}

@Override
public Integer next() {
return queue.poll();
}

@Override
public boolean hasNext() {
return !queue.isEmpty();
}
}
``````