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}.

代码如下：

 /**
  * Definition for singly-linked list.
  * public class ListNode {
  *     int val;
  *     ListNode next;
  *     ListNode(int x) { val = x; }
  * }
  */
 public class Solution {
     public void reorderList(ListNode head) {

     try{
         if(head.next.next!=null){
              ListNode p=head;

              Map<ListNode,ListNode> map=new HashMap<>();

             while(p.next.next!=null)
             {
                 map.put(p.next,p);
                 p=p.next;
             }
              ListNode q=head;
              ListNode temp=p.next;

              while(q.next!=p.next&&q.next!=null)
              {
                  p.next=temp.next;
                  temp.next=q.next;
                  q.next=temp;
                  q=temp.next;

                  temp=p;
                  p=map.get(p);
              }

         }

     }catch(NullPointerException e){}

     }
 }