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;
    }
};