Flatten Nested List Iterator.java

public class NestedIterator implements Iterator<Integer> {
private Stack<NestedInteger> stack;
    public NestedIterator(List<NestedInteger> nestedList) {
        stack = new Stack<>();
        for (int i = nestedList.size() - 1; i >= 0; i--) {
            stack.push(nestedList.get(i));
        }
    }

    @Override
    public Integer next() {
        if (hasNext()) {
            return stack.pop().getInteger();
        }
        throw new IllegalStateException("No more elements");
    }

    @Override
    public boolean hasNext() {
        while (!stack.isEmpty()) {
            NestedInteger top = stack.peek();
            if (top.isInteger()) {
                return true;
            }
            stack.pop();
            List<NestedInteger> nestedList = top.getList();
            for (int i = nestedList.size() - 1; i >= 0; i--) {
                stack.push(nestedList.get(i));
            }
        }
        return false;
    }
}