１４８链表排序

 归并排序：

public ListNode merge(ListNode head1, ListNode head2){
        ListNode temp = new ListNode(-1);
        ListNode cur = temp;
        while(head1 != null && head2 != null){
            if(head1.val <= head2.val){
                cur.next = head1;
                head1 = head1.next;
            }
            else{
                cur.next = head2;
                head2 = head2.next;
            }
            cur = cur.next;
        }
        if(head1 != null){
            cur.next = head1;
        }
        if(head2 != null){
            cur.next = head2;
        }
        return temp.next;
    }

    public ListNode sortList(ListNode head, ListNode tail){
        if(head == null ){
            return head;
        }

        //切掉尾巴,分成两个数组
        if(head.next == tail){
            head.next = null;
            return head;
        }
        ListNode slow =  head;
        ListNode fast = head;
        while(fast != tail){
            slow = slow.next;
            fast = fast.next;
            if( fast != tail)  fast = fast.next;
        }
        ListNode mid = slow;
        ListNode head1 = sortList(head, mid);
        ListNode head2 = sortList(mid, tail);
        ListNode result = merge(head1, head2);
        return  result;
    }

    public ListNode sortList(ListNode head) {
        return sortList(head, null);
    }

时间　ｏ（ｎｌｏｇｎ）　空间O（ｌｏｇｎ）

P．Ｓ．：链表一般会ｎｅｗ一个新节点指向ｈｅａｄ节点，当ｈｅａｄ节点发生变动的时候方便处理。另外一定要多判断是不是空指针，尤其是有next的时候。