每天一算：Swap Nodes in Pairs

 1// 24. Swap Nodes in Pairs
 2// https://leetcode.com/problems/swap-nodes-in-pairs/description/
 3// 时间复杂度: O(n)
 4// 空间复杂度: O(1)
 5class Solution {
 6public:
 7    ListNode* swapPairs(ListNode* head) {
 8
 9        ListNode* dummyHead = new ListNode(0);
10        dummyHead->next = head;
11
12        ListNode* p = dummyHead;
13        while(p->next && p->next->next){
14            ListNode* node1 = p->next;
15            ListNode* node2 = node1->next;
16            ListNode* next = node2->next;
17            node2->next = node1;
18            node1->next = next;
19            p->next = node2;
20            p = node1;
21        }
22
23        ListNode* retHead = dummyHead->next;
24        delete dummyHead;
25
26        return retHead;
27    }
28};