【LeetCode练习题】Swap Nodes in Pairs

class Solution {
public:
    ListNode *swapPairs(ListNode *head) {
        if(head == NULL)
            return NULL;
        ListNode *dummy = new ListNode(0);
        dummy->next = head;
        ListNode *pre = dummy,*first = head,*second = head->next;
        
        if(second == NULL){
            //只有一个节点
            delete dummy;
            return head;
        }
        else{
            //先交换第一个和第二个节点
            pre->next = second;
            first->next = second->next;
            second->next = first;
        }

        while(true){
            if(first->next == NULL || first->next->next == NULL)
                //first后面没有节点或者只有一个节点
                break;
            else{
                //有两个节点
                pre = first;
                first = first->next;
                second = first->next;

                pre->next = second;
                first->next = second->next;
                second->next = first;
            }
        }
        head = dummy->next;
        delete dummy;
        return head;
    }
};