143. 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：    把链表从中间分成两个链表，后半部分链表反序，然后重组链表*//** * Definition for singly-linked list. * struct ListNode { *     int val; *     struct ListNode *next; * }; */void reorderList(struct ListNode* head){    if(!head || !head->next) return ;struct ListNode *firstPart=head;struct ListNode *secondPart=head;struct ListNode *fast=head,*mid=head;while(fast->next && fast->next->next)//把链表分成两部分fast=fast->next->next,mid=mid->next;secondPart=mid->next;mid->next=NULL;reverseList(&secondPart);//把第二部分链表反序fast=head,mid=secondPart;struct ListNode *tmpf,*tmpm;while(mid)  //重新连接成一个链表{tmpf=fast->next;fast->next=mid;fast=tmpf;tmpm=mid->next;mid->next=tmpf;mid=tmpm;}}void reverseList(struct ListNode** head){if(!(*head) || !(*head)->next)return ;struct ListNode **h=head;struct ListNode *cur=*h,*pre;pre=cur->next;while(pre){cur->next=pre->next;pre->next=*h;*h=pre;pre=cur->next;}}