Interview Question for Software Development Engineer In Test (SDET) at Microsoft:

Assuming the list are in ascending order

public Node Merge(Node l1, Node l2)
        {
            if (l1 == null) return l2;
            if (l2 == null) return l1;

            Node root = new Node();
            Node first = root;

            while (l1 != null && l2 != null)
            {
                Node swap = (l1.Data < l2.Data) ? l1 : l2;
                root = root.Next = swap;
                l1 = l1 == swap ? l1.Next : l1;
                l2 = l2 == swap ? l2.Next : l2;
            }

            root.Next = (l2 == null) ? l1 : l2;
            return first.Next;
        }