[LeetCode] Swap Nodes in Pairs

Problem:

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

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

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

Solution:O(n)

public class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode n1 = dummy;
        ListNode n2 = head;
        
        while(n2!=null && n2.next!=null){
            ListNode temp = n2.next.next;
            n2.next.next = n1.next;
            n1.next = n2.next;
            n2.next = temp;
            n1 = n2;
            n2 = n1.next;
        }
        return dummy.next;
    }
}

Key Concept: Use a dummp Node 0 and then do correct swap sequence.