Sort a linked list using insertion sort.

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */public class Solution {    //插入排序，时间复杂度O(n^2),注意终止条件，默认p之前的已经从小到大排好序了    public ListNode insertionSortList(ListNode head) {        if(head==null||head.next==null) return head;        ListNode newHead=new ListNode(-1);        newHead.next=head;        ListNode p=head.next;        ListNode ppre=head;        boolean flag=false;        while(p!=null){
   ///          if(p.val>ppre.val){
   //              p=p.next;              ppre=ppre.next;          }else{              ListNode q=newHead.next;              ListNode qpre=newHead;              while(q.val