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