24. Swap Nodes in Pairs - Medium

Given a linked list, swap every two adjacent nodes and return its head.

You may not modify the values in the list’s nodes, only nodes itself may be changed.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Code

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head == null || head.next == null) return head;
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        head = dummy;
        while(head.next != null && head.next.next != null){
            ListNode n1 = head.next;
            ListNode n2 = head.next.next;
            n1.next = n2.next;
            n2.next = n1;
            head.next = n2;
            
            head = n1;
        }
        return dummy.next;
    }
}