[LeetCode] Reorder List

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You must do this in-place without altering the nodes' values.

For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.

思路：找到中间节点，将链表的后半部分就地逆置。然后插入到前半部分。

　　　时间复杂度O(n),空间复杂度O(1) 1 /**

 * 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) {
        if (head == NULL || head->next == NULL || head->next->next == NULL) 
            return;
        
        int length = 0;
        for (ListNode *pter = head; pter != NULL; pter = pter->next) {
            ++length;
        }
        
        int half_length = 1;
        ListNode *half = head;
        while (half_length < (length / 2)) {
            ++half_length;
            half = half->next;
        }
        
       //reverse
       ListNode *pter = half->next;
       half->next = NULL;
       while (pter != NULL) {
           ListNode *next_node = pter->next;
           pter->next = half->next;
           half->next = pter;
           pter = next_node;
       }
      

      ListNode *pter2 = head;
      pter = half->next;
      while (pter2 != half) {
          ListNode *next_node = pter->next;
          half->next =next_node;
          pter->next = pter2->next;
          pter2->next = pter;
          pter2 = pter2->next->next;
          pter = next_node;
      }
    }
};