Linked List - Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head. The values inside the nodes must not be modified — only node rearrangement is allowed.

Input: head = [1,2,3,4]
Output: [2,1,4,3]
Explanation: Pairs (1,2) and (3,4) swapped.

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

Use a dummy head. For each pair: link dummy→second, second→first, first→next pair start. Advance dummy by 2.

dummy.next = second; second.next = first; first.next = next pair
Advance: dummy = first (now behind next pair)

public ListNode swapPairs(ListNode head) { ListNode dummy = new ListNode(0); dummy.next = head; ListNode curr = dummy; while (curr.next != null && curr.next.next != null) { ListNode first = curr.next, second = curr.next.next; first.next = second.next; second.next = first; curr.next = second; curr = first; } return dummy.next; }

Complexity Analysis:

Time complexity: O(n)
Space complexity: O(1)