Swap Nodes in Pairs

Walk a linked list two nodes at a time and swap each pair of neighbours, then return the new head. The swap has to rewire the next pointers; copying values between nodes is not allowed.

Example: 1->2->3->4 becomes 2->1->4->3.

We could just iterate through the list and swap curr.val with curr.next.val. This is easy and perfectly fine in many real-world scenarios, but interviewers explicitly ban it to test your pointer manipulation skills.

To swap nodes A and B (prev -> A -> B -> next), we need to change 3 pointers:

1. prev.next should point to B.
2. A.next should point to next.
3. B.next should point to A.

Recursive: Swap the first two nodes, then recursively call the function for the rest of the list.
Iterative: Use a dummy node and a prev pointer to handle swaps in a loop.

Watch the three pointer assignments that swap each pair, one step each. Values never move — every node keeps its number from the first step to the last — only next pointers change.