0024: Swap Nodes in Pairs

Problem Statement

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]

Explanation:

Example 2:

Input: head = []

Output: []

Example 3:

Input: head = [1]

Output: [1]

Example 4:

Input: head = [1,2,3]

Output: [2,1,3]

Constraints:

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

Code Solution

class Solution:
    def swapPairs(self, head):
        
        dummy = ListNode(0, head)
        
        it = dummy
        while it and it.next and it.next.next:
            first = it.next
            second = it.next.next
            
            first.next = second.next
            second.next = first
            it.next = second
            
            it = it.next.next
        
        return dummy.next