[LeetCode]Reorder List

 1 class Solution {
 2 public:
 3     void reorderList(ListNode *head) {
 4         
 5         if (head == NULL || head->next == NULL || head->next->next == NULL) {return;}
 6         struct ListNode *fast = head;
 7         struct ListNode *slow = head;
 8 
 9          while(fast != NULL && fast->next != NULL) {
10             fast = fast->next->next;
11             slow = slow->next;
12         }
13 
14         //cout <<slow->val;
15         struct ListNode *CurPos = slow->next;
16         slow->next = NULL;
17 
18         //cout << CurPos->val;
19         //cout <<fast->val;
20         //反转链表
21         struct ListNode *PreCurPos = NULL;
22         struct ListNode *NextCurPos = CurPos->next;
23 
24         while(CurPos != NULL) {
25             CurPos->next = PreCurPos;
26             //cout<<CurPos->val<<endl;
27             PreCurPos = CurPos;
28             if (NextCurPos == NULL) {
29                 break;
30             }
31             CurPos = NextCurPos;
32             NextCurPos = CurPos->next;
33         }
34         slow = head;
35         head = merge(slow, CurPos);
36     }
37 
38     ListNode *merge(ListNode *lhs, ListNode *rhs) {
39         ListNode *newhead = new ListNode(0);
40         ListNode *p = newhead;
41         int flag = 0;
42         while (lhs && rhs) {
43             if (flag == 0) {
44                 p->next = lhs;
45                 lhs = lhs->next;
46                 flag = 1;
47             } else {
48                 p->next = rhs;
49                 rhs = rhs->next;
50                 flag = 0;
51             }
52             p = p->next;
53         }
54         if (lhs == NULL) {
55             p->next = rhs;
56         } else {
57             p->next = lhs;
58         }
59         p = newhead->next ;
60         newhead->next = NULL;
61         delete newhead;
62         return p;
63     }
64 };