Sort List

Sort a linked list in O(n log n) time using constant space complexity.

思路：

归并排序。题目要求固定空间，不知道这种递归实现的算不算固定空间。

代码：

    ListNode *sortList(ListNode *head, int length){
        if(length <= 1)
            return head;
        int l = length/2;
        ListNode *tmp = head;
        int num = 0;
        ListNode *half = NULL;
        while(tmp){
            num++;
            if(num == l){
                break;
            }
            tmp = tmp->next;
        }
        half = tmp->next;
        tmp->next = NULL;
        ListNode *first = sortList(head, l), *second = sortList(half, length-l);
        ListNode *result = NULL, *newhead;
        while(first&&second){
            if(first->val < second->val){
                if(result){
                    result->next = first;
                    result = result->next;
                }
                else{
                    result = first;
                    newhead = result;
                }
                first = first->next;
            }
            else{
                if(result){
                    result->next = second;
                    result = result->next;
                }
                else{
                    result = second;
                    newhead = result;
                }
                second = second->next;
            }
        }
        if(first)
            result->next = first;
        if(second)
            result->next = second;
        return newhead;
    }
    ListNode *sortList(ListNode *head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(head == NULL || head->next == NULL)
            return head;
        int length = 0;
        ListNode *tmp = head;
        while(head){
            length++;
            head = head->next;
        }
        return sortList(tmp, length);
    }

 第一遍归并排序是从中间分开的，还可以把链表按单双节点分别构成新链表。