Flatten Nested List Iterator LeetCode solution in Java

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public class NestedIterator implements Iterator<Integer> {
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  List<Integer> list = new ArrayList();
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  void flatten(List<NestedInteger> nestedList) {
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    for (NestedInteger nested : nestedList) {
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      if (nested.isInteger()) list.add(nested.getInteger());
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      else flatten(nested.getList());
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    }
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  }
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  public NestedIterator(List<NestedInteger> nestedList) {
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    flatten(nestedList);
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  }
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  int index = 0;
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  @Override
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  public Integer next() {
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    return list.get(index++);
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  }
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  @Override
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  public boolean hasNext() {
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    return index < list.size();
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  }
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}