Reorder List

 1 #define NULL 0
 2 
 3 class Solution {
 4 public:
 5     void reorderList(ListNode *head) {
 6         stack<ListNode*> s;
 7         ListNode* ptr1,*ptr2,*tmp;
 8         if(head != NULL && head->next != NULL && head->next->next != NULL)
 9         {
10            ptr1 = head;
11            ptr2 = head;
12            while(ptr2->next != NULL && ptr2->next->next != NULL)
13            {
14                ptr1 = ptr1->next;
15                ptr2 = ptr2->next->next;
16            }
17            tmp = ptr1;
18            ptr1 = ptr1->next;
19            tmp->next = NULL;
20            ptr2 = head;
21            while(ptr1 != NULL)
22            {
23                s.push(ptr1);
24                ptr1 = ptr1->next;
25            }
26            while(s.size())
27            {
28                tmp = ptr2->next;
29                ListNode* top;
30                top = s.top();
31                s.pop();
32                ptr2->next = top;
33                top->next = tmp;
34                ptr2 = ptr2->next->next;
35            }
36         }
37 
38     }
39 };