剑指offer36：两个链表的第一个公共结点

1 题目描述

　　输入两个链表，找出它们的第一个公共结点。

2 思路和方法

　　方法一：

　　用两个指针同时从两个链表的表头开始走，当走到自己的链表结尾的时候开始从另一个链表的表头开始向后走。终止条件就是两个指针第一次相遇。此时指针位置即为所求。（两个链表的节点和是一定的，所以两个指针一定可以同时遍历完两条链表，即在最后时刻两个指针一定是重合的）

　　方法2：

　　先数出两条链表的长度，得到长度差d，先将长链表从头结点往后走d步，之后第二个链表从头开始，两个链表一起一步一步走，直到两个链表的节点第一次相等为止，此时指针位置即为所求。

3 C++核心代码

ListNode* FindFirstCommonNode( ListNode *pHead1, ListNode *pHead2) {
        ListNode *p1 = pHead1;
        ListNode *p2 = pHead2;
        while(p1!=p2){
            p1 = (p1==NULL ? pHead2 : p1->next);
            p2 = (p2==NULL ? pHead1 : p2->next);
        }
        return p1;
    }

    ListNode* FindFirstCommonNode( ListNode* pHead1, ListNode* pHead2) {
        ListNode *p1=pHead1;
        ListNode *p2=pHead2;
        int len1=0,len2=0,diff=0;
        while(p1!=NULL){
            p1=p1->next;
            len1++;
        }
        while(p2!=NULL){
            p2=p2->next;
            len2++;
        }
        if(len1>len2){
            diff=len1-len2;
            p1=pHead1;
            p2=pHead2;
        }
        else{
            diff=len2-len1;
            p1=pHead2;
            p2=pHead1;
        }
        for(int i=0;i<diff;i++){
            p1=p1->next;
        }
        while(p1!=NULL && p2!=NULL){
            if(p1==p2)
                break;
            p1=p1->next;
            p2=p2->next;
        }
        return p1;
    }

4 C++完整代码

#include<iostream>
using namespace std;
 
struct ListNode
{
    int m_nValue;
    ListNode* m_pNext;
};
 
//创建链表节点
ListNode* CreateListNode(int value)
{
    ListNode* pNode = new ListNode();
    pNode->m_nValue = value;
    pNode->m_pNext = NULL;
 
    return pNode;
}
 
 
//连接链表节点
void ConnectListNodes(ListNode* pCurrent, ListNode* pNext)
{
    if (pCurrent == NULL)
    {
        //exit(1)，非正常运行导致退出程序；exit(0)，正常运行并退出程序
        cout << "Error to connect two nodes." << endl;
        exit(1);
    }
 
    pCurrent->m_pNext = pNext;
}
 
 
//销毁链表
void DestroyList(ListNode* pHead)
{
    ListNode* pNode = pHead;
 
    while (pNode!=NULL)
    {
        pHead = pHead->m_pNext;
        delete pNode;
        pNode = pHead;
    }
}
 
//销毁节点
void DestroyNode(ListNode* pNode)
{
    delete pNode;
    pNode = NULL;
}
 
//求链表的长度
unsigned int GetListLength(ListNode* pHead)
{
    unsigned int nLength = 0;
    ListNode* pNode = pHead;
 
    while (pNode!=NULL)
    {
        nLength++;
        pNode = pNode->m_pNext;
    }
 
    return nLength;
}
 
//找第一个公共节点
ListNode* FindFirstCommonNode(ListNode *pHead1, ListNode *pHead2)
{
    //求两个链表的长度
    unsigned int nLength1 = GetListLength(pHead1);
    unsigned int nLength2 = GetListLength(pHead2);
    //两个链表的长度差
    int nLengthDif = nLength1 - nLength2;
    ListNode* pListHeadLong = pHead1;
    ListNode* pListHeadShort= pHead2;
    if (nLength2 > nLength1)
    {
        pListHeadLong = pHead2;
        pListHeadShort = pHead1;
        nLengthDif = nLength2 - nLength1;
    }
 
    // 先在长链表上走几步，再同时在两个链表上遍历
    for (int i = 0; i < nLengthDif; i++)
    {
        pListHeadLong = pListHeadLong->m_pNext;
    }
 
    while ((pListHeadLong!=NULL)&& (pListHeadShort != NULL)&&(pListHeadLong != pListHeadShort))
    {
        pListHeadLong = pListHeadLong->m_pNext;
        pListHeadShort = pListHeadShort->m_pNext;
    }
 
    //得到第一个公共节点
    ListNode* pFisrtCommonNode = pListHeadLong;
    //ListNode* pFisrtCommonNode = pListHeadShort;也可以
 
    return pFisrtCommonNode;
}
 
 
// ====================测试代码====================
void Test(char* testName, ListNode* pHead1, ListNode* pHead2, ListNode* pExpected)
{
    if (testName != NULL)
    {
        cout << testName << "  begins: ";
    }
 
    ListNode* pResult = FindFirstCommonNode(pHead1, pHead2);
    if (pResult == pExpected)
    {
        cout << "Succeed!" << endl;
    }
    else
    {
        cout << "Failed!" << endl;
    }
}
 
 
// 第一个公共结点在链表中间
// 1 - 2 - 3 
//                6 - 7
//      4 - 5 /
void Test1()
{
    ListNode* pNode1 = CreateListNode(1);
    ListNode* pNode2 = CreateListNode(2);
    ListNode* pNode3 = CreateListNode(3);
    ListNode* pNode4 = CreateListNode(4);
    ListNode* pNode5 = CreateListNode(5);
    ListNode* pNode6 = CreateListNode(6);
    ListNode* pNode7 = CreateListNode(7);
 
    ConnectListNodes(pNode1, pNode2);
    ConnectListNodes(pNode2, pNode3);
    ConnectListNodes(pNode3, pNode6);
    ConnectListNodes(pNode4, pNode5);
    ConnectListNodes(pNode5, pNode6);
    ConnectListNodes(pNode6, pNode7);
 
    Test("Test1", pNode1, pNode4, pNode6);
 
    DestroyNode(pNode1);
    DestroyNode(pNode2);
    DestroyNode(pNode3);
    DestroyNode(pNode4);
    DestroyNode(pNode5);
    DestroyNode(pNode6);
    DestroyNode(pNode7);
}
 
// 没有公共结点
// 1 - 2 - 3 - 4
//            
// 5 - 6 - 7
void Test2()
{
    ListNode* pNode1 = CreateListNode(1);
    ListNode* pNode2 = CreateListNode(2);
    ListNode* pNode3 = CreateListNode(3);
    ListNode* pNode4 = CreateListNode(4);
    ListNode* pNode5 = CreateListNode(5);
    ListNode* pNode6 = CreateListNode(6);
    ListNode* pNode7 = CreateListNode(7);
 
    ConnectListNodes(pNode1, pNode2);
    ConnectListNodes(pNode2, pNode3);
    ConnectListNodes(pNode3, pNode4);
    ConnectListNodes(pNode5, pNode6);
    ConnectListNodes(pNode6, pNode7);
 
    Test("Test2", pNode1, pNode5, NULL);
 
    DestroyList(pNode1);
    DestroyList(pNode5);
}
 
// 公共结点是最后一个结点
// 1 - 2 - 3 - 4 
//                      7
//           5 - 6 /
void Test3()
{
    ListNode* pNode1 = CreateListNode(1);
    ListNode* pNode2 = CreateListNode(2);
    ListNode* pNode3 = CreateListNode(3);
    ListNode* pNode4 = CreateListNode(4);
    ListNode* pNode5 = CreateListNode(5);
    ListNode* pNode6 = CreateListNode(6);
    ListNode* pNode7 = CreateListNode(7);
 
    ConnectListNodes(pNode1, pNode2);
    ConnectListNodes(pNode2, pNode3);
    ConnectListNodes(pNode3, pNode4);
    ConnectListNodes(pNode4, pNode7);
    ConnectListNodes(pNode5, pNode6);
    ConnectListNodes(pNode6, pNode7);
 
    Test("Test3", pNode1, pNode5, pNode7);
 
    DestroyNode(pNode1);
    DestroyNode(pNode2);
    DestroyNode(pNode3);
    DestroyNode(pNode4);
    DestroyNode(pNode5);
    DestroyNode(pNode6);
    DestroyNode(pNode7);
}
 
// 公共结点是第一个结点
// 1 - 2 - 3 - 4 - 5
// 两个链表完全重合   
void Test4()
{
    ListNode* pNode1 = CreateListNode(1);
    ListNode* pNode2 = CreateListNode(2);
    ListNode* pNode3 = CreateListNode(3);
    ListNode* pNode4 = CreateListNode(4);
    ListNode* pNode5 = CreateListNode(5);
 
    ConnectListNodes(pNode1, pNode2);
    ConnectListNodes(pNode2, pNode3);
    ConnectListNodes(pNode3, pNode4);
    ConnectListNodes(pNode4, pNode5);
 
    Test("Test4", pNode1, pNode1, pNode1);
 
    DestroyList(pNode1);
}
 
// 输入的两个链表有一个空链表
void Test5()
{
    ListNode* pNode1 = CreateListNode(1);
    ListNode* pNode2 = CreateListNode(2);
    ListNode* pNode3 = CreateListNode(3);
    ListNode* pNode4 = CreateListNode(4);
    ListNode* pNode5 = CreateListNode(5);
 
    ConnectListNodes(pNode1, pNode2);
    ConnectListNodes(pNode2, pNode3);
    ConnectListNodes(pNode3, pNode4);
    ConnectListNodes(pNode4, pNode5);
 
    Test("Test5", NULL, pNode1, NULL);
 
    DestroyList(pNode1);
}
 
// 输入的两个链表都是空链表
void Test6()
{
    Test("Test6", NULL, NULL, NULL);
}
 
 
int main()
{
    Test1();
    Test2();
    Test3();
    Test4();
    Test5();
    Test6();
    system("pause");
    return 0;
}

参考资料

https://blog.csdn.net/lingfeng2019/article/details/80778598

https://blog.csdn.net/yanxiaolx/article/category/6250534（完整代码）