Implement the complete reorder in space.
Key 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: . Space: .