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
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）
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