面试题56之链表中环的入口结点_EntryNodeInListLoop.cpp

/* 
 * Question Description:
 * (Question 17 in <Coding Intervies>) If there is a loop in a linked list, how do you get the entry node of the loop? 
 * The entry node is the first node in the loop from the head of a list.
*/

#include <stdio.h>
#include "Utilities/list.h"

ListNode* MeetingNode(ListNode* pHead)
{
    if(pHead == NULL)
        return NULL;

    ListNode* pSlow = pHead->m_pNext;
    if(pSlow == NULL)
        return NULL;

    ListNode* pFast = pSlow->m_pNext;
    while(pFast != NULL && pSlow != NULL)
    {
        if(pFast == pSlow)
            return pFast;

        pSlow = pSlow->m_pNext;

        pFast = pFast->m_pNext;
        if(pFast != NULL)
            pFast = pFast->m_pNext;
    }

    return NULL;
}

ListNode* EntryNodeOfLoop(ListNode* pHead)
{
    ListNode* meetingNode = MeetingNode(pHead);
    if(meetingNode == NULL)
        return NULL;

    // get the number of nodes in loop
    int nodesInLoop = 1;
    ListNode* pNode1 = meetingNode;
    while(pNode1->m_pNext != meetingNode)
    {
        pNode1 = pNode1->m_pNext;
        ++nodesInLoop;
    }

    // move pNode1
    pNode1 = pHead;
    for(int i = 0; i < nodesInLoop; ++i)
        pNode1 = pNode1->m_pNext;

    // move pNode1 and pNode2
    ListNode* pNode2 = pHead;
    while(pNode1 != pNode2)
    {
        pNode1 = pNode1->m_pNext;
        pNode2 = pNode2->m_pNext;
    }

    return pNode1;
}

// ==================== Test Code ====================
void Test(char* testName, ListNode* pHead, ListNode* entryNode)
{
    if(testName != NULL)
        printf("%s begins: ", testName);

    if(EntryNodeOfLoop(pHead) == entryNode)
        printf("Passed.\n");
    else
        printf("FAILED.\n");
}

// A list has a node, without a loop
void Test1()
{
    ListNode* pNode1 = CreateListNode(1);

    Test("Test1", pNode1, NULL);

    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 = NULL;
}

// 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 = NULL;
    delete pNode2;
    pNode2 = NULL;
    delete pNode3;
    pNode3 = NULL;
    delete pNode4;
    pNode4 = NULL;
    delete pNode5;
    pNode5 = NULL;
}

// 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 = NULL;
    delete pNode2;
    pNode2 = NULL;
    delete pNode3;
    pNode3 = NULL;
    delete pNode4;
    pNode4 = NULL;
    delete pNode5;
    pNode5 = NULL;
}

// 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 = NULL;
    delete pNode2;
    pNode2 = NULL;
    delete pNode3;
    pNode3 = NULL;
    delete pNode4;
    pNode4 = NULL;
    delete pNode5;
    pNode5 = NULL;
}

// 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, NULL);

    DestroyList(pNode1);
}

// Empty list
void Test7()
{
    Test("Test7", NULL, NULL);
}

int main(int argc, char* argv[])
{
    Test1();
    Test2();
    Test3();
    Test4();
    Test5();
    Test6();
    Test7();

	return 0;
}