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

reverse_nodes_in_swap.py

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def swapPairs(self, head: 'ListNode') -> 'ListNode':
        if not head or not head.next:
            return head

        cur = head
        prev = None
        nex = cur.next
        
        while cur and cur.next:
            nex_nex = cur.next.next
            cur.next.next = cur
            if prev:
                prev.next = cur.next
            cur.next = nex_nex
            prev = cur
            cur = nex_nex
        
        return nex