// 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 <iostream>
#include "List.h"ListNode* MeetingNode(ListNode* pHead)//鲁棒一：先确定有没有环，有的话先求得环中任意一个节点
{if (pHead == nullptr)//若头结点为空return nullptr;ListNode* pSlow = pHead->m_pNext;if (pSlow == nullptr)//若只有一个节点return nullptr;ListNode* pFast = pSlow->m_pNext;while (pFast != nullptr && pSlow != nullptr)//判断碰到尾节点后有没有环
    {if (pFast == pSlow)return pFast;pSlow = pSlow->m_pNext;pFast = pFast->m_pNext;if (pFast != nullptr)//判断碰到尾节点后有没有环pFast = pFast->m_pNext;}return nullptr;
}ListNode* EntryNodeOfLoop(ListNode* pHead)//开始计算入口节点，第一步先求环的个数，第二步通过两个前后指针计算入口节点
{ListNode* meetingNode = MeetingNode(pHead);if (meetingNode == nullptr)//如果存在环，得到一个环中节点return nullptr;// 得到环中结点的数目int nodesInLoop = 1;ListNode* pNode1 = meetingNode;while (pNode1->m_pNext != meetingNode){pNode1 = pNode1->m_pNext;++nodesInLoop;}// 先移动pNode1，次数为环中结点的数目pNode1 = pHead;for (int i = 0; i < nodesInLoop; ++i)pNode1 = pNode1->m_pNext;// 再移动pNode1和pNode2ListNode* pNode2 = pHead;while (pNode1 != pNode2)//二者只能相遇在入口处
    {pNode1 = pNode1->m_pNext;pNode2 = pNode2->m_pNext;}return pNode1;
}// ==================== Test Code ====================
void Test(const char* testName, ListNode* pHead, ListNode* entryNode)
{if (testName != nullptr)printf("%s begins: ", testName);if (EntryNodeOfLoop(pHead) == entryNode)printf("Passed.\n");elseprintf("FAILED.\n");
}// 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();system("pause");return 0;
}