Reorder List

先将链表用快慢指针分成两部分，再将后一半进行倒置，倒置后再合并。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 
 ListNode* reverse(ListNode *head)
{
    if(head==NULL||head->next==NULL)return head;
    ListNode prehead(0),*p;
    prehead.next=head;
    while(head->next)
    {
        p=head->next;
        head->next=p->next;
        p->next=prehead.next;
        prehead.next=p;
    }
    
    return prehead.next;
}

class Solution {
public:
    ListNode *reorderList(ListNode* head) {
      if(head==NULL||head->next==NULL)return head;
      ListNode prehead(0),*slower,*fast,*start;
      prehead.next=head;
      slower=fast=head;
      while(fast&&fast->next)
      {
          slower=slower->next;
          fast=fast->next->next;
      }
      start=slower->next;
      slower->next=NULL;
     start=reverse(start);
     while(head&&start)
     {
        fast=start->next;
        start->next=head->next;
        head->next=start;
        
        head=head->next->next;
        start=fast;
     }
     
     return prehead.next; 
      
        
    }
};