24. Swap Nodes in Pairs¶

Edited by Charles

Medium · Python · LinkedList

Problem

The official page contains the problem statement, examples, and constraints.

Explanation, examples & other approaches

Core idea

This question requires to return a whole linked list, and thus a virtual head is needed. The pointer current should start from the virtual head, since the next and the next.next are two objects for swapping.

Original study notes follow, including intermediate snippets and earlier attempts.

This question requires to return a whole linked list, and thus a virtual head is needed.
The pointer current should start from the virtual head, since the next and the next.next are two objects for swapping.
The while loop condition should be the to-be-swapped nodes are not None.
1. VirtualHead → Node[0] → Node[1] → Node[2] → ……
```
       |                     |                                         |

 current           slow                                fast
```
The swap looks like
1. VirtualHead Node[0] → Node[1] → Node[2] → ……
```
        |_______________________|
```
2. VirtualHead Node[0] ← Node[1] → Node[2] → ……
```
        |_______________________|
```
3. |—————>————|
  
  VirtualHead Node[0] ← Node[1] Node[2] → ……
```
  |_______________________|
```
4. VirtualHead → Node[1] → Node[0] → Node[2] → ……
Besides the pointer current, we also need two pointers: slow and fast, since by redirecting the nodes, we need to find nodes that lost original references. For example, in step 4, Node[0] and Node[2] need alternative references, since without new references, we cannot find them after redirecting VirtualHead to Node[1] and redirecting Node[1] to Node[0].

Study record

Field	Value
Attempts	2
Mastery	❚❚
The Latest Attempt	January 4, 2026

Python solution¶

# 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: Optional[ListNode]) -> Optional[ListNode]:
        dummy = ListNode(next = head)
        current = dummy

        while current.next and current.next.next:
            slow = current.next
            fast = current.next.next.next

            current.next = current.next.next
            current.next.next = slow
            slow.next = fast

            current = current.next.next

        return dummy.next

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