Sort a linked list using insertion sort.

一刷
题解：没有很好的办法，只能每次从头找。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode insertionSortList(ListNode head) {
        if(head == null) return head;
        
        ListNode helper = new ListNode(0);
        ListNode cur = head; //the node will be inserted
        ListNode pre = helper; //insert node between pre and pre.next
        ListNode next = null;//the next node will be inserted
        
        while(cur!=null){
            next = cur.next;
            while(pre.next!=null && pre.next.val<cur.val) pre = pre.next;
            
            //insert between pre and per.next
            
            cur.next = pre.next;
            pre.next = cur;
            pre = helper;
            cur = next;
        }
        
        return helper.next;
    }
}