148. Sort List LeetCode

题意：给一个链表排序。
题解：对链表快排？我选择归并，归并好写一些。

class Solution {
public:
    ListNode* getmid(ListNode* head)
    {
        ListNode* fast,*slow;
        fast = head,slow = head;
        while(fast->next != NULL && fast->next->next != NULL)
        {
            fast = fast->next->next;
            slow = slow->next;
        }
        return slow;
    }
    ListNode* merge(ListNode* head1,ListNode* head2)
    {
        ListNode* head = new ListNode(0);
        ListNode* ans = head;
        while(head1 && head2)
        {
            if(head1->val < head2->val)
            {
                head->next = head1;
                head1 = head1->next;
            }
            else
            {
                head->next = head2;
                head2 = head2->next;
            }
            head = head->next;
        }
        head->next = head1 == NULL ? head2 : head1;
        return ans->next;
    }
    ListNode* sortList(ListNode* head) {
        if(head == NULL || head->next == NULL)
            return head;
        ListNode* mid = getmid(head);
        ListNode* nxt = mid->next;
        mid->next=  NULL;
        return merge(sortList(head),sortList(nxt));
    }
};