Sort List

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

思路：使用O(nlogn)时间复杂度和常数空间复杂度，我们想到可以用归并排序。

1）找到链表中间位置

2）将两个链表按序合并链表

3）对所给链表进行整体的归并排序

/**

 * Definition for singly-linked list.

 * struct ListNode {

 *     int val;

 *     ListNode *next;

 *     ListNode(int x) : val(x), next(NULL) {}

 * };

 */

class Solution {

public:

    ListNode *getMiddle(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 *head1,ListNode *head2)

    {

        ListNode *dummy=new ListNode(-1);

        ListNode *cur=dummy;

        while(head1!=NULL&&head2!=NULL)

        {

            if(head1->val<head2->val)

            {

                cur->next=head1;

                head1=head1->next;

            }

            else

            {

                cur->next=head2;

                head2=head2->next;

            }

            cur=cur->next;

        }

        while(head1!=NULL)

        {

            cur->next=head1;

            head1=head1->next;

            cur=cur->next;

        }

        while(head2!=NULL)

        {

            cur->next=head2;

            head2=head2->next;

            cur=cur->next;

        }

        return dummy->next;

    }

    ListNode *sortList(ListNode *head) {

        if(head==NULL||head->next==NULL)

            return head;

        ListNode *mid=getMiddle(head);

        ListNode *pHead2=mid->next;

        mid->next=NULL;

        return Merge(sortList(head),sortList(pHead2));

    }

};