Swap Nodes in Pairs – Solution & Complexity
Solution Walkthrough
1. Recognize the Pattern
The key pattern is: a dummy-node walk that repoints three links at a time — prev, the first node of the pair, and the second node of the pair. Identify the state and invariant before coding.
2. Build the Algorithm
Advance one state transition at a time. Mark or update state before exploring dependent work.
3. Check Edge Cases
Test empty or minimal input, skewed shapes, duplicates where allowed, and impossible outcomes.
4. Solution and Complexity
Time: O(n) — every node is visited once. Space: O(1) auxiliary for the iterative Python/JavaScript/Java/Go solutions. The Rust solution here is written recursively for idiomatic ownership handling of Option<Box<ListNode>> swaps, so it costs O(n) call-stack space instead.
All 5 languages below run and submit against the remote judge for this problem — Java/Go/Rust use the same real ListNode structural-type support the judge added for the rest of the linked-list/tree track.
