思路 使用归并排序

public class Solution{
    public ListNode sortList(ListNode head){
        //终止条件
        if(head==null||head.next==null)
            return head;
        ListNode prev = null,slow=head,fast=head;
        //分为前后两部分
        while(fast!=null&&fast.next!=null){
            prev = slow;
            slow = slow.next;
            fast = fast.next.next;
        }
        prev.next = null;
        //分别对前后两部分进行排序
        ListNode l1 = sortList(head);
        ListNode l2 = sortList(slow);
        //对两部分进行合并
        return merge(l1,l2);
    }
    ListNode merge(ListNode l1,ListNode l2){
        ListNode dummyHead = new ListNode(0),p=dummyHead;
        //合并的过程
        while(l1!=null&&l2!=null){
            if(l1.val<l2.val){
                p.next = l1;
                l1 = l1.next;
            }else{
                p.next = l2;
                l2 = l2.next;
            }
            p = p.next;
        }
        if(l1!=null)
            p.next = l1;
        if(l2!=null)
            p.next = l2;
        return dummyHead.next;
    }
}