力扣——两两交换链表中的节点

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head==null)
            return head;
        ListNode preHead=new ListNode(-1);
        preHead.next=head;
        ListNode left=preHead;
        ListNode mid=head;
        if(head.next==null)//如果只有一个节点，直接返回这个节点
            return head;
        ListNode right=head.next;
        while(mid!=null&&mid.next!=null){
            mid.next=right.next;
            right.next=mid;
            left.next=right;
        
            left=mid;
            mid=left.next;
            if(mid!=null)
            right=mid.next;
        }
        return preHead.next;
    }
}