Implement the merge using the dummy head technique.
Watch for:
- 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
tailafter 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: .