Reorganizing Linked Lists

Reorganizing Linked Lists does not specify a unique transformation. First ask for the desired output on a small example: are nodes being reordered by position, partitioned by value, merged from multiple lists, sorted, or grouped by parity? Confirm whether node identity must be preserved, whether new nodes are allowed, and whether the list may contain cycles. Restate the transformation before choosing an algorithm.

For a positional reorder, a common plan is midpoint plus reverse plus alternating merge. For a stable partition, maintain dummy heads for the two groups and concatenate them. For merging sorted lists, repeatedly choose the smaller head; for sorting, use merge sort because linked lists split and merge efficiently. In every version, save next before relinking and keep an invariant describing the completed prefix. Test empty, singleton, two-node, all-one-group, duplicate values, and odd lengths. Complexity depends on the confirmed task: stable partition and merge are commonly O(n) time with O(1) auxiliary pointer space, while linked-list merge sort is O(n log n) time. Do not claim one answer until the ordering rule is known.