sort-list

class Solution {
public:
    ListNode *sortList(ListNode *head) {
        if(!head||!head->next)return head;
        ListNode*slow=head,*fast=head->next;
        while(fast&&fast->next)//这里必须是fast=head->next，考虑偶数时slow的最终位置
        {
            slow=slow->next,fast=fast->next->next;
        }
        ListNode*l1=sortList(slow->next);
        slow->next=NULL;
        ListNode*l2=sortList(head);
        return mergelist(l1,l2);
    }
    ListNode*mergelist(ListNode*l1,ListNode*l2)
    {
        ListNode*dummy=new ListNode(-1);
        ListNode*root=dummy;
        while(l1&&l2)
        {
            if(l1->val<l2->val)root->next=l1,l1=l1->next,root=root->next;
            else root->next=l2,l2=l2->next,root=root->next;
        }
        if(l1)root->next=l1;
        if(l2)root->next=l2;
        return dummy->next;
    }
};