6 ms java solution with Stack, O(k) space complexity - k is the maximum depth of nested list

  • 0

    Store the next value in 'val'. If next value is not available, 'val' is null. Using a Deque as a Stack to store the nested iterators. If the next value is a list, push the current iterator to the stack, continue searching. Else if next is Integer, set it to 'val' and break;

    public class NestedIterator implements Iterator<Integer> {
        Integer val = null;
        Iterator<NestedInteger> iter = null;
        Deque<Iterator<NestedInteger>> queue = new ArrayDeque<> ();
        public NestedIterator(List<NestedInteger> nestedList) {
            iter = nestedList.iterator();
        public Integer next() {
            Integer tmp = val;
            val = null;
            while (iter.hasNext() || !queue.isEmpty()) {
                if (iter.hasNext()) {
                    NestedInteger next = iter.next();
                    if (next.isInteger()) {
                        val = next.getInteger();
                    } else {
                        iter = next.getList().iterator();
                } else 
                    iter = queue.pop();
            return tmp;
        public boolean hasNext() {
            return val != null;

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