【Leetcode】【Medium】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}.

解题思路：

设置一个指针mid指向链表的中间结点；

将mid之后的半个链表进行反转，反转后的链表insert;

将insert中的结点挨个插入前半段链表中；

代码：

 /**
  * 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 || !head->next || !head->next->next)
             return;
         ListNode* ctrl = head;
         ListNode* mid = head;
         ListNode* insert = NULL;

         while (ctrl->next && ctrl->next->next) {
             ctrl = ctrl->next->next;
             mid = mid->next;
         }

         insert = reverseList(mid->next);
         mid->next = NULL;
         ctrl = head;

         while (insert) {
             ListNode* t = ctrl->next;
             ctrl->next = insert;
             insert = insert->next;
             ctrl->next->next = t;
             ctrl = t;
         }

         return;
     }

     ListNode* reverseList(ListNode* head) {
         if (!head->next)
             return head;
         ListNode* cur = NULL;
         ListNode* next = head;
         ListNode* left = NULL;
         while (next) {
             left = next->next;
             next->next = cur;
             cur = next;
             next = left;
         }
         return cur;
     }
 };