[leetcode] Swap Nodes in Pairs

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode* swapPairs(ListNode* head) {
12         if(NULL==head || NULL==head->next) return head;
13         ListNode *mock_head = new ListNode(-1);
14         mock_head->next=head;
15         ListNode *flag_node=mock_head;
16         while(flag_node) {
17             vector<ListNode*> nodes;
18             do {
19                 nodes.push_back(flag_node);
20                 flag_node=flag_node->next;
21             } while (nodes.size()<3 && flag_node);
22             nodes.push_back(flag_node);
23             if (nodes.size()<4) break;
24             nodes[0]->next = nodes[2];
25             nodes[1]->next = nodes[3];
26             nodes[2]->next = nodes[1];
27             flag_node = nodes[1];
28         }
29         return mock_head->next;
30     }
31 };