Sort List 归并排序链表

Sort List

 

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

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public://归并排序 快慢取中间值    ListNode* sortList(ListNode* head) {                if(head==NULL || head->next==NULL)            return head;       ListNode *mid=getMid(head);       ListNode *next=mid->next;       mid->next=NULL;       return merge(sortList(head),sortList(next));      }        ListNode *getMid(ListNode *head)    {        ListNode *slow=head;        ListNode *fast=head;               while(fast->next!=NULL&&fast->next->next!=NULL)        {            slow=slow->next;            fast=fast->next->next;        }        return slow;    }        ListNode *merge(ListNode *a,ListNode *b)    {        ListNode *help=new ListNode(0);        ListNode *cur=help;        while(a!=NULL && b!=NULL)        {            if(a->val<b->val)            {                cur->next=a;                a=a->next;            }else            {                cur->next=b;                b=b->next;            }            cur=cur->next;        }        cur->next=(a!=NULL)?a:b;        return help->next;    }};