输入： head = [1,2,3,4]
输出： [2,1,4,3]

/**
 * 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) {
          ListNode dum = new ListNode(0);
        dum.next = head;
        ListNode curr = dum;
        while (curr.next != null && curr.next.next != null) {
            ListNode node1 = curr.next;
            ListNode node2 = curr.next.next;
            curr.next=node2;
            node1.next=node2.next;
            node2.next=node1;
            curr=node1;
        }
        return dum.next;
    }
}