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24. Swap Nodes in Pairs

Difficulty Topics

Description

Given a linked list, swap every two adjacent nodes and return its head. You must solve the problem without modifying the values in the list's nodes (i.e., only nodes themselves may be changed.)

 

Example 1:

Input: head = [1,2,3,4]
Output: [2,1,4,3]

Example 2:

Input: head = []
Output: []

Example 3:

Input: head = [1]
Output: [1]

 

Constraints:

  • The number of nodes in the list is in the range [0, 100].
  • 0 <= Node.val <= 100

Solution

swap-nodes-in-pairs.py
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def swapPairs(self, head: ListNode) -> ListNode:
        res = curr = ListNode(-1, head)

        while curr.next and curr.next.next:
            first, second = curr.next, curr.next.next
            following = curr.next.next.next

            curr.next, second.next, first.next = second, first, following

            curr = first

        return res.next