public int next(){
        Node node = null;
        while(root != null){
            stack.push(root);
            root = root.left;
        }
        root = stack.pop();
        node = root;
        root = root.right;
        return node.data;
    }
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