Merge Two Sorted Lists Solution

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 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: $O(n)$. Space: $O(1)$.