链表的归并排序（经典实现：递归）

 ListNode* MergeSort(ListNode* head)
 {
	 if(head == NULL || head->next == NULL) return head;

	 ListNode *head1 = head, *head2 = head;
	 while(head2->next != NULL &&  head2->next->next != NULL)
	 {
		 head1 = head1->next;
		 head2 = head2->next->next;
	 }
	 head2 = head1->next;
	 head1->next = NULL;
	 head1 = head;

	 head1 = MergeSort(head1);
	 head2 = MergeSort(head2);

	 //merge
	 ListNode *guard = new ListNode();
	 ListNode *cur = guard;
	 while(head1 != NULL && head2 != NULL)
	 {
		 if(head1->val <= head2->val) 
		 {
			 cur->next = head1;
			 cur = cur->next;
			 head1 = head1->next;
		 }
		 else
		 {
			 cur->next = head2;
			 cur = cur->next;
			 head2 = head2->next;
		 }
	 }
	 if(head1 != NULL) cur->next = head1;
	 if(head2 != NULL) cur->next = head2;

	 head = guard->next;
	 delete guard;
	 return head;
 }