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].

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * public interface NestedInteger {
 *
 *     // @return true if this NestedInteger holds a single integer, rather than a nested list.
 *     public boolean isInteger();
 *
 *     // @return the single integer that this NestedInteger holds, if it holds a single integer
 *     // Return null if this NestedInteger holds a nested list
 *     public Integer getInteger();
 *
 *     // @return the nested list that this NestedInteger holds, if it holds a nested list
 *     // Return null if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
public class NestedIterator implements Iterator<Integer> {
    private LinkedList<NestedInteger> stack;

    public NestedIterator(List<NestedInteger> nestedList) {
        stack = new LinkedList<>();
        pushToStack(nestedList);
    }

    @Override
    public Integer next() {
        return hasNext() ? stack.pollFirst().getInteger() : null;
    }

    @Override
    public boolean hasNext() {
        while (!stack.isEmpty()) {
            NestedInteger cur = stack.peekFirst();
            if (cur.isInteger()) {
                return true;
            }
            pushToStack(stack.pollFirst().getList());
        }
        return false;
    }
    
    private void pushToStack(List<NestedInteger> nestedList) {
        // add in reverse order to make pop easy
        for (int i = nestedList.size() - 1; i >= 0; i--) {
            stack.offerFirst(nestedList.get(i));
        }
    }
}

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i = new NestedIterator(nestedList);
 * while (i.hasNext()) v[f()] = i.next();
 */