知识储备：03链表：链表之哑节点的使用

要对头结点进行操作时，考虑创建哑节点dummy，使用dummy->next表示真正的头节点。这样可以避免处理头节点为空的边界问题。

1.给定一个链表和x，对链表排序使所有小于x的节点出现在大于或等于x的节点的前面。

代码如下：

#include 

using namespace std;

/*
链表之哑节点的使用
2-17-8-22
*/

struct ListNode{
	int value;
	ListNode *next;
	ListNode(int x){ value = x; }
};

//排序：小与x都在大于或等于x的前面
ListNode *reorderXList(ListNode *head, int x)
{
	ListNode *newhead = NULL;
	ListNode *aDummy = new ListNode(0);
	ListNode *aCurr = aDummy;
	ListNode *bDummy = new ListNode(0);
	ListNode *bCurr = bDummy;

	while (head)
	{
		ListNode *next = head->next;
		head->next = NULL;
		if (head->value < x)
		{
			aCurr->next = head;
			aCurr = head;//后移
		}
		else
		{
			bCurr->next = head;
			bCurr = head;
		}
		head = next;
	}

	aCurr->next = bDummy->next;//连接起来
	newhead = aDummy->next;//新的头节点

	delete aDummy;
	delete bDummy;

	return newhead;
}

int main()
{
	ListNode *a1, *a2, *a3, *a4;
	ListNode *head;
	head = (ListNode*)malloc(sizeof(ListNode));
	a1 = (ListNode*)malloc(sizeof(ListNode));
	a2 = (ListNode*)malloc(sizeof(ListNode));
	a3 = (ListNode*)malloc(sizeof(ListNode));
	a4 = (ListNode*)malloc(sizeof(ListNode));

	head->next = a1;
	a1->value = 5;
	a1->next = a2;
	a2->value = 8;
	a2->next = a3;
	a3->value = 3;
	a3->next = a4;
	a4->value = 6;
	a4->next = NULL;

	ListNode *resHead;
	resHead = reorderXList(head, 6);
	
	return 0;
}