学步园

输入两个链表，找出他们的第一个公共结点：

struct ListNode
{
	int value;
	struct ListNode *next;
};

unsigned int GetListLength(ListNode *pHead)
{
	ListNode *pNode = pHead;
	unsigned int length = 0;
	while (pNode != NULL)
	{
		length++;
		pNode = pNode->next;
	}
	return length;
}

ListNode *FindFirstCommonNode(ListNode *pHead1, ListNode *pHead2)
{
	unsigned int nLength1 = GetListLength(pHead1);
	unsigned int nLength2 = GetListLength(pHead2);
	int nLengthDif = nLength1 - nLength2;
	
	ListNode *pListHeadLong = pHead1;
	ListNode *pListHeadShort = pHead2;
	if (nLengthDif < 0)
	{
		pListHeadLong = pHead2;
		pListHeadShort = pHead1;
		nLengthDif = -nLengthDif;
	}

	for (unsigned int i = 0; i < nLengthDif; i++)
	{
		pListHeadLong = pListHeadLong->next;
	}
	while ((pListHeadLong != NULL) && (pListHeadShort != NULL) && (pListHeadShort != pListHeadLong))
	{
		pListHeadLong = pListHeadLong->next;
		pListHeadShort = pListHeadShort->next;
	}
	
	ListNode *pFirstCommon = pListHeadLong;
	return pFirstCommon;
}