[LeetCode]148. Sort List链表归并排序

public ListNode sortList(ListNode head) {
        if (head==null||head.next==null) return head;
        //利用快慢指针找到链表中点，并分成两部分递归
        //要设置一个超前节点记录慢指针前一个节点作为前部分末尾
        ListNode slow = head,fast = head,pre = null;
        while (fast!=null&&fast.next!=null)
        {
            pre = slow;
            slow = slow.next;
            fast = fast.next.next;
        }
        //前半部分
        pre.next = null;
        ListNode l1 = sortList(head);
        ListNode l2 = sortList(slow);
        return merge(l1,l2);
    }
    public ListNode merge(ListNode l1,ListNode l2)
    {
        //设置超前节点
        ListNode pre = new ListNode(0),temp = pre;
        while (l1!=null&&l2!=null)
        {
            if (l1.val<l2.val)
            {
                temp.next = l1;
                l1 = l1.next;
            }
            else {
                temp.next = l2;
                l2 = l2.next;
            }
            temp = temp.next;
        }
        if (l1!=null) temp.next = l1;
        if (l2!=null) temp.next = l2;
        return pre.next;
    }