C++算法之 找出两个链表的公共节点

题目：输入两个链表，找出它们第一个公共节点。链表节点定义如下：

struct ListNode

｛

int    m_nKey;

ListNode*   m_pNext;

｝

方法1：在第一个链表上顺序遍历每一个节点，没遍历一个节点，在第二个链表上顺序遍历每个节点。  O(n^2)

方法2：找到两个链表的长度差，先遍历长链表到短链表的长度处，然后两个链表同时遍历，没遍历依次比较两个节点指针是否相同，

注意是比较节点指针，不是节点的值！

代码：

// FindFirstCommandNode.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include <iostream>
using namespace std;

struct ListNode
{
	int         m_nKey;
	ListNode*   m_pNext;

	ListNode(int i):m_nKey(i)
	{

	}
};

//获取链表长度
int GetListLength(ListNode* pHead)
{
	int nLength = 0;
	ListNode* pNode = pHead;
	while (pNode != NULL)
	{
		++nLength;
		pNode = pNode->m_pNext;
	}
	return nLength;
}

ListNode* FindFirstCommandNode(ListNode* pHead1, ListNode* pHead2)
{

	int  nLength1 = GetListLength(pHead1);
	int  nLength2 = GetListLength(pHead2);
	int nLengthDif = 0;//两个链表的长度差
	ListNode* pListHeadLong  = NULL;//用于指向长链表
	ListNode* pListHeadShort = NULL;//用于指向短链表

	//根据长度判断 链表指向
	if (nLength1 > nLength2)
	{
	    nLengthDif = nLength1 - nLength2;
		pListHeadShort = pHead2;
		pListHeadLong  = pHead1;
	}
	else
	{
		nLengthDif = nLength2 - nLength1;
		pListHeadLong  = pHead2;
		pListHeadShort = pHead1;
	}

	//先对长链表进行移动 移动到与短链表长度相同的位置
	for (int i = 0; i < nLengthDif; i++)
	{
		pListHeadLong = pListHeadLong->m_pNext;
	}
	//寻找公共节点
	while (pListHeadLong !=NULL && pListHeadShort != NULL && pListHeadLong!= pListHeadShort)
	{
		pListHeadLong  = pListHeadLong->m_pNext;
		pListHeadShort = pListHeadShort->m_pNext;
	}
	//如果不为空  此时的pListHeadLong 与pListNodeShort为同一个节点，返回该节点
	if (pListHeadLong != NULL)
	{
		return pListHeadLong;
	}
	else
	{
		return NULL;//否则返回NULL
	}
}

int _tmain(int argc, _TCHAR* argv[])
{

	ListNode* head1 = new ListNode(0);
	ListNode* head2 = new ListNode(1);
	ListNode* node0 = new ListNode(22);
	ListNode* node1 = new ListNode(2);
	ListNode* node2 = new ListNode(3);
	ListNode* node3 = new ListNode(4);
	ListNode* node4 = new ListNode(5);
	ListNode* node5 = new ListNode(6);
	ListNode* node6 = new ListNode(7);
	ListNode* node8 = new ListNode(6);

	head1->m_pNext = node1;
	node1->m_pNext = node0;
	node0->m_pNext = node3;
	node3->m_pNext = node5;
	node5->m_pNext = node6;
	node6->m_pNext = NULL;

	head2->m_pNext = node2;
	node2->m_pNext = node4;
	node4->m_pNext = node8;
	node8->m_pNext = node6;
	node6->m_pNext = NULL;

	cout<<"链表1的长度为："<<GetListLength(head1)<<endl;
	cout<<"链表2的长度为："<<GetListLength(head2)<<endl;

	ListNode* CommNode = FindFirstCommandNode(head1,head2);
	if (CommNode!= NULL)
	{
		cout<<"公共节点的值为："<<CommNode->m_nKey<<endl;
	}
	else
	{
		cout<<"没有公共节点"<<endl;
	}
	getchar();
	return 0;
}