Reorder List Solution

Implement the complete reorder in $O(1)$ space.

Implementation details:

• Be careful with the middle: for even lengths, you want the first half to be one node longer
• After reversing, second points to the new head of the reversed half
• In the merge phase, save next pointers before overwriting

This problem is a checkpoint.

If you can solve it cleanly, you've internalized the core linked list techniques. Test on: even length, odd length, two nodes, single node. Time: $O(n)$. Space: $O(1)$.