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  • 《剑指offer》第二十三题:链表中环的入口结点

    // 面试题23:链表中环的入口结点
    // 题目:一个链表中包含环,如何找出环的入口结点?例如,在图3.8的链表中,
    // 环的入口结点是结点3。
    
    #include <cstdio>
    #include "list.h"
    
    ListNode* MeetingNode(ListNode* pHead)
    {
        if (pHead == nullptr) //空节点
            return nullptr;
    
        ListNode* pSlow = pHead->m_pNext; //单节点
        if (pSlow == nullptr)
            return nullptr;
    
        ListNode* pFast = pSlow->m_pNext;
       
        while (pFast != nullptr && pSlow != nullptr)
        {
            if (pFast == pSlow)
                return pFast;
    
            pSlow = pSlow->m_pNext;
            pFast = pFast->m_pNext;
            if (pFast != nullptr) //快节点走两步
                pFast = pFast->m_pNext;
        }
        return nullptr;
    }
    
    ListNode* EntryNodeOfLoop(ListNode* pHead)
    {
        //1.首先遍历链表查找是否存在环
        ListNode* meetingNode = MeetingNode(pHead);
        if (meetingNode == nullptr)
            return nullptr;
    
        //2.然后算出环的长度n
        int length = 1;
        ListNode* pNode = meetingNode;
        while (pNode->m_pNext != meetingNode)
        {
            pNode = pNode->m_pNext;
            ++length;
        }
    
        //3.接着指针1先走n步
        ListNode* pNodeFast = pHead;
        for (int i = 0; i < length; ++i)
            pNodeFast = pNodeFast->m_pNext;
    
        //4.最后指针12同时往前遍历, 相等时即入口
        pNode = pHead;
        while (pNode != pNodeFast)
        {
            pNode = pNode->m_pNext;
            pNodeFast = pNodeFast->m_pNext;
        }
        return pNode;
    }
    // ==================== Test Code ====================
    void Test(const char* testName, ListNode* pHead, ListNode* entryNode)
    {
        if (testName != nullptr)
            printf("%s begins: ", testName);
    
        if (EntryNodeOfLoop(pHead) == entryNode)
            printf("Passed.
    ");
        else
            printf("FAILED.
    ");
    }
    
    // A list has a node, without a loop
    void Test1()
    {
        ListNode* pNode1 = CreateListNode(1);
    
        Test("Test1", pNode1, nullptr);
    
        DestroyList(pNode1);
    }
    
    // A list has a node, with a loop
    void Test2()
    {
        ListNode* pNode1 = CreateListNode(1);
        ConnectListNodes(pNode1, pNode1);
    
        Test("Test2", pNode1, pNode1);
    
        delete pNode1;
        pNode1 = nullptr;
    }
    
    // A list has multiple nodes, with a loop 
    void Test3()
    {
        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);
        ConnectListNodes(pNode5, pNode3);
    
        Test("Test3", pNode1, pNode3);
    
        delete pNode1;
        pNode1 = nullptr;
        delete pNode2;
        pNode2 = nullptr;
        delete pNode3;
        pNode3 = nullptr;
        delete pNode4;
        pNode4 = nullptr;
        delete pNode5;
        pNode5 = nullptr;
    }
    
    // A list has multiple nodes, with a loop 
    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);
        ConnectListNodes(pNode5, pNode1);
    
        Test("Test4", pNode1, pNode1);
    
        delete pNode1;
        pNode1 = nullptr;
        delete pNode2;
        pNode2 = nullptr;
        delete pNode3;
        pNode3 = nullptr;
        delete pNode4;
        pNode4 = nullptr;
        delete pNode5;
        pNode5 = nullptr;
    }
    
    // A list has multiple nodes, with a loop 
    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);
        ConnectListNodes(pNode5, pNode5);
    
        Test("Test5", pNode1, pNode5);
    
        delete pNode1;
        pNode1 = nullptr;
        delete pNode2;
        pNode2 = nullptr;
        delete pNode3;
        pNode3 = nullptr;
        delete pNode4;
        pNode4 = nullptr;
        delete pNode5;
        pNode5 = nullptr;
    }
    
    // A list has multiple nodes, without a loop 
    void Test6()
    {
        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("Test6", pNode1, nullptr);
    
        DestroyList(pNode1);
    }
    
    // Empty list
    void Test7()
    {
        Test("Test7", nullptr, nullptr);
    }
    
    int main(int argc, char* argv[])
    {
        Test1();
        Test2();
        Test3();
        Test4();
        Test5();
        Test6();
        Test7();
    
        return 0;
    }
    测试代码

    分析:双指针解决。 

    /*
    struct ListNode {
        int val;
        struct ListNode *next;
        ListNode(int x) :
            val(x), next(NULL) {
        }
    };
    */
    class Solution {
    public:
        ListNode* EntryNodeOfLoop(ListNode* pHead)
        {
            //1.寻找环
            ListNode* meetingNode = MeetingNode(pHead);
            if (meetingNode == nullptr)
                return nullptr;
            
            //2.确定环长n
            int length = 1;
            ListNode* pNode = meetingNode;
            while (pNode->next != meetingNode)
            {
                pNode = pNode->next;
                ++length;
            }
            
            //3.指针1先走n步
            pNode = pHead;
            for (int i = 0; i < length; ++i)
                pNode = pNode->next;
                
            //4.指针12同时遍历,相遇时即为起点
            ListNode* pNode2 = pHead;
            while (pNode != pNode2)
            {
                pNode = pNode->next;
                pNode2 = pNode2->next;
            }
            return pNode;
        }
        ListNode* MeetingNode(ListNode* pHead)
        {
            if (pHead == nullptr)
                return nullptr;
            
            ListNode* pSlow = pHead->next;
            if (pSlow == nullptr)
                return nullptr;
            
            ListNode* pFast = pSlow->next;
            while (pFast != nullptr && pSlow != nullptr)
            {
                if (pFast == pSlow)
                    return pFast;
                
                pSlow = pSlow->next;
                pFast = pFast->next;
                if (pFast != nullptr)
                    pFast = pFast->next;
            }
            return nullptr;
        }
    };
    牛客网提交代码
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  • 原文地址:https://www.cnblogs.com/ZSY-blog/p/12577990.html
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