143. Reorder List

First, the topic

1, examining

　　

2. Analysis

Give a list of one-way lists, reorder them, and sort the rules as above.

Second, the answer

1, Ideas:

Method One,

Watch the list of reordered links above. Can be divided into three steps.

①, find the middle node;

②, flip the linked list in the back part.

③, inserts a node in the linked list in the front part of the list.

     Public voidreorderlist (ListNode head) {if(Head = =NULL|| Head.next = =NULL)            return; ListNode Walker=Head; ListNode Runner=Head; //1. Find Mid         while(Runner.next! =NULL&& Runner.next.next! =NULL) {Walker=Walker.next; Runner=Runner.next.next; } ListNode Midnode=Walker; //2. Reverse PostListNode pre =Midnode.next; Midnode.next=NULL; ListNode Next=NULL; if(Pre.next! =NULL) Next=Pre.next; Pre.next=NULL;  while(Pre! =NULL&& Next! =NULL) {ListNode Nextandnext=Next.next; Next.next=Pre; Pre=Next; Next=Nextandnext; }                //3. Insert NodesListNode next2 =NULL;  while(Head! =NULL&& Pre! =NULL) {Next=Head.next; Next2=Pre.next; Head.next=Pre; Pre.next=Next; Head=Next; Pre=next2; }    }

143. Reorder List