public Node connect(Node root) {
    if(root == null) return root;
    Deque<Node> q = new LinkedList();
    q.offer(root);
    while(!q.isEmpty()) {
        int size = q.size();
        for(int i = 0; i < size; i++) {
            Node node1 = q.poll();
            if(node1.left != null) q.offer(node1.left);
            if(node1.right != null) q.offer(node1.right);
            if(i != size -1) {
                Node node2 = q.peek();
                node1.next =node2;
            }
            else {
                node1.next = null;
            }
        }
    }
    return root;
}