LeetCode Sort List（連結清單排序）

題目連結

struct ListNode *MergeSortList(struct ListNode *head1, struct ListNode *head2)
{
	struct ListNode *p1, *p2, *p, *temp;
	p = (struct ListNode *)malloc(sizeof(struct ListNode));
	temp = p;				//暫存合并後的連結清單的頭結點
	p1 = head1;
	p2 = head2;
	while (p1&&p2)
	{
		if (p1->val <= p2->val)
		{
			p->next = p1;
			p1 = p1->next;
		}
		else
		{
			p->next = p2;
			p2 = p2->next;
		}
		p = p->next;
	}
	if (p1)
		p->next = p1;
	else
		p->next = p2;
	p = temp->next;
	return p;
}

struct ListNode *MergeSort(struct ListNode *head)
{
	struct ListNode *fast, *slow, *slowPrev, *leftHalf, *rightHalf;
	fast = slow = head;
	if (!head || !head->next)
		return head;
	while (fast&&fast->next)
	{
		fast = fast->next->next;
		slowPrev = slow;
		slow = slow->next;
	}
	slowPrev->next = NULL;
	leftHalf = MergeSort(head);
	rightHalf = MergeSort(slow);
	return MergeSortList(leftHalf, rightHalf);
}

struct ListNode *sortList(struct ListNode *head)		
{
	if (!head || !head->next)
		return head;
	return MergeSort(head);				//歸并排序，保證算法複雜度為O(nlog(n))
}