public boolean hasNext(){
        if(root != null || stack.size() > 0){
            return true;
        }
        return false;
    }
http://www.glassdoor.com/Interview/Create-an-iterator-to-traverse-a-binary-tree-When-the-next-function-is-called-on-the-binary-tree-return-the-value-at-the-QTN_674695.htm
null tree.
src/com/interview/tree/NextInorderSuccessorIterator.java