Sort List——链表的快速/归并排序

用O(nlgn)时间对单链表排序，常量空间

参考：http://blog.youkuaiyun.com/xudli/article/details/16819545

思路：排序——O(nlgn)——》快排或者归并，选择归并，将链表分成两等分进行归并排序

ListNode *mergeSort(ListNode *head){//
	ListNode *p=head;
	ListNode *q=head;
	ListNode *pre=head;
	if(!p || !p->next) return head;
	
	while(p && p->next){
		p = p->next->next;
		pre = q;
		q = q->next;
	}
	pre->next = NULL;
	ListNode *h1 = mergeSort(head);
	ListNode *h2 = mergeSort(q);
	return merge(h1, h2);
}


ListNode *merge(ListNode *h1, ListNode *h2){
	ListNode *temp = new ListNode(0);
	ListNode *p = temp;
	while(h1 && h2){
		if(h1->val < h2->val){
			p->next = h1;
			h1 = h1->next;
		}
		else{
			p->next = h2;
			h2 = h2->next;
		}
		p = p->next;
	}
	if(h1) p->next = h1;
	else p->next = h2;
	
	ListNode *h = temp->next;
	delete temp;
	return h;
}