LinkedIn: Deep Iterator

	Question:
	题目是这样的
	eg: {{1,2},3,{4,{5,6}}}
	不断调用iterator的next()返回的序列是 1 2 3 4 5 6
	这个data structure的interface是这样的
	public interface Data {
	// Does this Data hold a collection?
	public boolean isCollection();
	// Returns the collection contained by // this Data, or null if it is a single element
	public Collection> getCollection();
	// Returns the single element contained //by this Data, or null if it is collection
	public T getElement();
	}
	Code (Java):
	public class DeepIterator implements Iterator {
	private Stack> stack = new Stack<>();
	private Data top = null;
	public DeepIterator(List input) {
	stack.push(input.iterator());
	}
	@Override
	public boolean hasNext() {
	if (this.top != null) {
	return true;
	}
	while (!stack.isEmpty()) {
	Iterator it = stack.peek();
	if (it.hasNext()) {
	Data curr = it.next();
	if (!curr.isCollection()) {
	top = curr.getElement();
	return true;
	} else {
	stack.push(curr.getCollection.iterator());
	}
	} else {
	stack.pop();
	}
	}
	return false;
	}
	@Override
	public Integer next() {
	if (top != null) {
	Integer result = top;
	top = null;
	return result;
	} else {
	return null;
	}
	}
	}