L148 排序链表

链表

在 O(n log n) 时间复杂度和常数级空间复杂度下，对链表进行排序。示例 1:
输入: 4->2->1->3
输出: 1->2->3->4

解：O(NLOGN)的排序方法包括，快排，归并，堆。根据单链表的特点，最适合用于归并排序。

//归并排序
 ListNode* sortList(ListNode* head) {
        // special case
        if(!head || !head->next) return head;
        ListNode *slow = head,*fast= head, *cur = head;
        // find the middle of the link
        while(fast && fast->next){
            cur = slow;
            slow = slow->next;
            fast = fast -> next ->next;
        }
        cur->next = NULL;
        return merge(sortList(head),sortList(slow));
    }
    ListNode* merge(ListNode *l1, ListNode *l2){
        ListNode *dummy = new ListNode(-1);
        ListNode *cur = dummy;
        while(l1 && l2){
            if(l1->val < l2->val){
                cur->next = l1;
                l1 = l1->next;
            }else{
                cur->next = l2;
                l2 = l2->next;
            }
            cur = cur->next;
        }
        cur->next = l1?l1:l2;
        return dummy->next;
    }
//作弊sort
ListNode* sortList(ListNode* head) {
        if(head==NULL) return head;
        vector<int> res;
        ListNode *newhead=head;
        while(head->next!=NULL){
            res.push_back(head->val);
            head=head->next;
        }
        res.push_back(head->val);
        sort(res.begin(),res.end());
        head = newhead;
        int i=0;
        while(head->next!=NULL){
            head->val=res[i];
            head=head->next;
            i++;
        }
        head->val=res[i];
        return newhead;
    }