LetCode 24. 两两交换链表中的节点

static int dividend=[](){
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if (head == NULL)
            return NULL;
        if (head->next == NULL)
            return head;
        ListNode* prv = head;
        ListNode* mid = head;
        ListNode* last = head->next;
        while(1){
            if (prv != mid){
                prv->next = mid->next;
                mid->next = last->next;
                last->next = mid;
            }else{
                mid->next = last->next;
                last->next = mid;
                head = last;
            }
            
            if (mid->next == NULL || mid ->next->next == NULL)
                return head;
            else{
                prv = mid;
                last = mid->next->next;
                mid = mid->next;
            }
                
        }
    }
};