Trade Places in Twos

Medium

You are given a singly linked list head. Group its nodes into consecutive pairs (first with second, third with fourth, and so on) and swap the two nodes in every pair. If the number of nodes is odd, the last one stays where it is. Return the head of the resulting list. Move the nodes themselves by rewiring the next links; do not just swap the values stored in them.

Example 1

head=list_node([4, 8, 15, 16, 23]) → [8, 4, 16, 15, 23]

The pairs (4, 8) and (15, 16) trade places; 23 has no partner and stays at the end.

Example 2

head=list_node([0, 9]) → [9, 0]

The only pair trades places.

Example 3

head=list_node([]) → None

An empty list stays empty.

Constraints

The list has between 0 and 100 nodes
0 ≤ Node.val ≤ 100

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Topics

Recursion Linked List

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Input

head=list_node([4, 8, 15, 16, 23])

Expected

[8, 4, 16, 15, 23]

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