148. Sort List

Sort a linked list in O(n log n) time using constant space complexity.

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 ListNode sortList(ListNode head) {
         if(head==null||head.next==null)
         return head;

         ListNode p=head;
         int count=0;

         while(p!=null)
         {
             count++;
             p=p.next;
         }
         int[]a=new int[count];
         int c=count;
         p=head;
        for(int i=0;i<count;i++)
        {
            if(p!=null)
            a[i]=p.val;
            p=p.next;
        }
        p=head;

         Arrays.sort(a);
         for(int i=0;i<c;i++)
         {
             p.val=a[i];
             p=p.next;
         }
         return head;
     }
 }