leetcode——Swap Nodes in Pairs

class Solution {
public:
// 两个指针，p, q, q指向第二个,preP是p之前
    ListNode *swapPairs(ListNode *head) {
        if(head == NULL) return NULL;
        auto p = head;
        auto q = head->next;
        ListNode preHead(-1);
        ListNode* preP = &preHead;
        preP->next = p;
        ListNode* tmp;
        while(q){
            preP->next = q;
            tmp = q->next;// 记录q之后，作为下一个p
            q->next = p;
            preP = p;
            if(tmp) p = tmp;
            else break;
            q = p->next;
        }
        return preHead.next;
    }
};

class Solution {
public:
// 两个指针，p, q, q指向第二个,preP是p之前
    ListNode *swapPairs(ListNode *head) {
        if(head == NULL) return NULL;
        auto p = head;
        auto q = head->next;
        ListNode preHead(-1);
        ListNode* preP = &preHead;
        preP->next = p;
        while(q){
            preP->next = q;
            p->next = q->next;// 记录q之后，作为下一个p
            q->next = p;
            preP = p;
            p = p->next;
            q = p?p->next:nullptr;
        }
        return preHead.next;
    }
};