Implement the merge using the dummy head technique.
Key points: - The dummy head eliminates the "which node becomes the new head?" question - After the loop, exactly one list might have remaining nodes - You're reusing existing nodes, not creating new ones Common bugs: - Forgetting to advance tail after appending - Not handling when one list is initially empty - Creating new nodes instead of reusing existing ones This problem is a building block for merge sort on linked lists. Time: . Space: .