leetcode之Sort List

问题描述如下：

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

问题链接

cpp代码如下：

class Solution {
public:
    ListNode *sortList(ListNode *head) {
        if(head==NULL||head->next==NULL)return head;
        ListNode *s=head,*f=head;
        while(true){
            f=f->next;
            if(f==NULL)break;
            f=f->next;
            if(f==NULL)break;
            s=s->next;
        }
        f=s->next;
        s->next=NULL;
        s=sortList(head);
        f=sortList(f);
        if(s->val<=f->val){
            head=s;
            s=s->next;
        }else{
            head=f;
            f=f->next;
        }
        ListNode* tail=head;
        while(s!=NULL&&f!=NULL){
            if(s->val<=f->val){
                tail->next=s;
                tail=s;
                s=s->next;
            }else{
                tail->next=f;
                tail=f;
                f=f->next;
            }
        }
        if(s!=NULL)tail->next=s;
        else tail->next=f;
        return head;
    }
};