Reorder List

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    void reorderList(ListNode *head) {
        ListNode *first = head, *middle = head, *tail = head, *pre_tail;

     if (head == NULL || head->next == NULL) { return;}

     pre_tail = middle;
     // find the middle node
     while (tail != NULL && tail->next != NULL) {
         pre_tail = middle;
         middle = middle->next;
         if (tail->next->next != NULL) {
             tail = tail->next->next;
         } else {
             tail = tail->next;
         }
     }

     ListNode *prev = NULL, *current = middle, *next=middle->next;
     while (current != NULL) {
         current->next = prev;
         prev = current;

         current = next;
         if (next != NULL) {
             next = next->next;
         }
     }

     ListNode *subHead = prev, *subNext = prev;

     current = head;
     while (current != NULL) {
         next = current->next;

         current->next = subHead;
         current = next;

         if (subHead != NULL) {
             subNext = subHead->next;
             subHead->next = next;

             subHead = subNext;
         }
     }
    }
};