Swap Nodes in Pairs

Leetcode 24.

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/**
 * 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) {
       //head = [1,2,3,4] 
       ListNode newOne = new ListNode(-1);
       ListNode pre = newOne;
       //newOne = [-1,1,2,3,4]
       newOne.next = head; 
       while(pre.next != null && pre.next.next != null){
         ListNode l1 = pre.next;
         ListNode l2 = pre.next.next; 
         ListNode next = l2.next; 
         l1.next = next;
         l2.next = l1; 
         pre.next = l2; 
         pre = l1;   
       }

    return newOne.next; 
    }
}