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=,len2=,diff=;
         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=;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();
     }

     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 = ;
     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 = ; 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();
     ListNode* pNode2 = CreateListNode();
     ListNode* pNode3 = CreateListNode();
     ListNode* pNode4 = CreateListNode();
     ListNode* pNode5 = CreateListNode();
     ListNode* pNode6 = CreateListNode();
     ListNode* pNode7 = CreateListNode();

     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();
     ListNode* pNode2 = CreateListNode();
     ListNode* pNode3 = CreateListNode();
     ListNode* pNode4 = CreateListNode();
     ListNode* pNode5 = CreateListNode();
     ListNode* pNode6 = CreateListNode();
     ListNode* pNode7 = CreateListNode();

     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();
     ListNode* pNode2 = CreateListNode();
     ListNode* pNode3 = CreateListNode();
     ListNode* pNode4 = CreateListNode();
     ListNode* pNode5 = CreateListNode();
     ListNode* pNode6 = CreateListNode();
     ListNode* pNode7 = CreateListNode();

     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();
     ListNode* pNode2 = CreateListNode();
     ListNode* pNode3 = CreateListNode();
     ListNode* pNode4 = CreateListNode();
     ListNode* pNode5 = CreateListNode();

     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();
     ListNode* pNode2 = CreateListNode();
     ListNode* pNode3 = CreateListNode();
     ListNode* pNode4 = CreateListNode();
     ListNode* pNode5 = CreateListNode();

     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 ;
 }

参考资料

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

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