找出链表的第一个公共结点

题目：两个单链表，找出它们的第一个公共结点。

链表的结点定义为：

struct ListNode
{
int m_nKey;
ListNode* m_pNext;
};

答：

#include "stdafx.h"
#include <iostream>
#include <fstream>

using namespace std;

struct ListNode
{
int m_nKey;
ListNode* m_pNext;
};

//构造链表
void CreateList(ListNode *&pHead, fstream &fin, ListNode *pNodeCommon)
{
ListNode *pNode = NULL;
ListNode *pTmp = NULL;
int data;
fin>>data;
while (data)
{
pNode = new ListNode;
pNode->m_nKey = data;
pNode->m_pNext = NULL;
if (NULL == pHead)
{
pHead = pNode;
pTmp = pNode;
}
else
{
pTmp->m_pNext = pNode;
pTmp = pNode;
}

fin>>data;
}
if (NULL != pNode)
{
pNode->m_pNext = pNodeCommon;
}

}

//求链表长度
int ListLength(ListNode *pHead)
{
if (NULL == pHead)
{
return 0;
}
ListNode *pNode = pHead;
int length = 0;
while (NULL!= pNode)
{
length++;
pNode = pNode->m_pNext;
}
return length;
}

//找出链表的第一个公共结点
ListNode* FindFirstCommonNode(ListNode *pFirstHead, ListNode *pSecondHead)
{
ListNode *pShortNode = pFirstHead;
ListNode *pLongNode = pSecondHead;
int lengthShort = ListLength(pShortNode);
int lengthLong = ListLength(pLongNode);

if (lengthShort > lengthLong)   //1、求出两个链表的长度
{
int temp = lengthLong;
lengthLong = lengthShort;
lengthShort = temp;
ListNode *PTmp = pShortNode;
pShortNode = pLongNode;
pLongNode = PTmp;
}
int separate = lengthLong - lengthShort;

for (int i = 0; i < separate && NULL != pLongNode; i++)  //2、让长度长的链表先遍历两个链表的长度的差值
{
pLongNode = pLongNode->m_pNext;
}

while (NULL != pShortNode && NULL != pLongNode && (pShortNode != pLongNode))  //3、第一个相等的链表结点就是第一个公共结点
{
pShortNode = pShortNode->m_pNext;
pLongNode = pLongNode->m_pNext;
}
if (pShortNode == pLongNode)
{
return pShortNode;
}
return NULL;
}

int _tmain(int argc, _TCHAR* argv[])
{
fstream finShort("listShort.txt");
fstream finLong("listLong.txt");
fstream finCommon("listCommon.txt");
ListNode *pNodeShort = NULL;
ListNode *pNodeLong = NULL;
ListNode *pNodeCommon = NULL;
CreateList(pNodeCommon, finCommon, NULL);
CreateList(pNodeShort, finShort, pNodeCommon);
CreateList(pNodeLong, finLong, pNodeCommon);
ListNode *pFirstCommon = FindFirstCommonNode(pNodeShort, pNodeLong);
if (NULL != pFirstCommon)
{
cout<<"两个链表的第一个公共结点为："<<pFirstCommon->m_nKey;
}
else
{
cout<<"两个链表没有公共结点";
}
cout<<endl;
return 0;
}

运行界面如下：



建造长链表用到的listLong.txt为：

555 8 9 20 44 33 20 8 4 0

建造短链表用到的listShort.txt为：

7 16 39 0

建造公共链表用到的listCommon.txt为：

555 4 50 77 99 0