Swap Nodes in Pairs — Leetcode Pattern Recognition Practice

https://leetcode.com/problems/swap-nodes-in-pairs/

Problem: Swap every two adjacent nodes in a linked list and return its head.

Pattern

Linked List reversal pattern

This problem is essentially Reverse Nodes in k-Group where k = 2 for every group.

Once that clicks, the solution becomes very mechanical.

Intuition (My Way)

• Since swapping pairs means reversing every adjacent pair, I treated the list in chunks of size 2.
• Instead of writing a special-case solution, I reused the same iterative k-group reversal logic.
• A dummy node helps simplify edge cases and keeps pointer updates clean.

For every iteration:

• Find the 2nd node from the current start.
• Reverse the nodes between start.next and kth.next.
• Re-link the reversed part back to the main list.
• Move start forward to prepare for the next pair.

This avoids recursion and keeps everything pointer-based.

Complexity

• Time Complexity: O(n) — each node is visited once
• Space Complexity: O(1) — in-place reversal, no extra data structures

class Solution(object):
    def swapPairs(self, head)…