https://www.youtube.com/watch?app=desktop&v=377ZjTI2NoM

1 -> 2 -> 3 -> 4

2 1 4 3

0 -> 1 -> 2 -> 3 -> 4
l1	 l2       nextStart
0 -> 2 -> 1 -> 3 -> 4
l1        l2 
           
0 -> 2 -> 1 -> 3 -> 4
          l1  l2
class Solution {
	public ListNode swapPairs (ListNode head) {
		if (head == null || head.next == null) return head;
		ListNode dummy = new ListNode(0);
		dummy.next = head;
		ListNode l1 = dummy;
		ListNode l2 = head;
		while(l2 != null && l2.next != null) {
			ListNode nextStart = l2.next.next;
			l1.next = l2.next;
			l2.next.next = l2;
			l2.next = nextStart;
			l1 = l2;
			l2 = l2.next;
		}
		return dummy.next;	
	}
}