Flattening a Linked List

The problem we are exploring is to flatten a singly linked list. The initial singly linked list (to be flattened) has nodes of the following type:

This is a companion discussion topic for the original entry at http://iq.opengenus.org/flattening-a-linked-list/

can any one explain the time complexity? It sounds like o(n) as we are only traversing n nodes and each comparison seems to be a constant operations.