082_FirstCommonNodesInLists.cpp

// Coding Interviews: Questions, Analysis & Solutions
// Harry He

#include <stdio.h>
#include "..\Utility\List.h"

unsigned int GetListLength(ListNode* pHead);

ListNode* FindFirstCommonNode( ListNode *pHead1, ListNode *pHead2)
{
    // Get length of two lists
    unsigned int nLength1 = GetListLength(pHead1);
    unsigned int nLength2 = GetListLength(pHead2);
    int nLengthDif = nLength1 - nLength2;
 
    ListNode* pListHeadLong = pHead1;
    ListNode* pListHeadShort = pHead2;
    if(nLength2 > nLength1)
    {
        pListHeadLong = pHead2;
        pListHeadShort = pHead1;
        nLengthDif = nLength2 - nLength1;
    }
 
    // Move d steps on the longer list if the length difference is d
    for(int i = 0; i < nLengthDif; ++ i)
        pListHeadLong = pListHeadLong->m_pNext;
 
    while((pListHeadLong != NULL) && 
        (pListHeadShort != NULL) &&
        (pListHeadLong != pListHeadShort))
    {
        pListHeadLong = pListHeadLong->m_pNext;
        pListHeadShort = pListHeadShort->m_pNext;
    }
 
    // Traverse two lists
    ListNode* pFisrtCommonNode = pListHeadLong;
 
    return pFisrtCommonNode;
}

unsigned int GetListLength(ListNode* pHead)
{
    unsigned int nLength = 0;
    ListNode* pNode = pHead;
    while(pNode != NULL)
    {
        ++ nLength;
        pNode = pNode->m_pNext;
    }
 
    return nLength;
}

// ==================== Test Code ====================
void DestroyNode(ListNode* pNode);

void Test(char* testName, ListNode* pHead1, ListNode* pHead2, ListNode* pExpected)
{
    if(testName != NULL)
        printf("%s begins: ", testName);

    ListNode* pResult = FindFirstCommonNode(pHead1, pHead2);
    if(pResult == pExpected)
        printf("Passed.\n");
    else
        printf("Failed.\n");
}

// 1 - 2 - 3 \
//            6 - 7
//     4 - 5 /
void Test1()
{
    ListNode* pNode1 = CreateListNode(1);
    ListNode* pNode2 = CreateListNode(2);
    ListNode* pNode3 = CreateListNode(3);
    ListNode* pNode4 = CreateListNode(4);
    ListNode* pNode5 = CreateListNode(5);
    ListNode* pNode6 = CreateListNode(6);
    ListNode* pNode7 = CreateListNode(7);

    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);
}

// No intersection
// 1 - 2 - 3 - 4
//            
// 5 - 6 - 7
void Test2()
{
    ListNode* pNode1 = CreateListNode(1);
    ListNode* pNode2 = CreateListNode(2);
    ListNode* pNode3 = CreateListNode(3);
    ListNode* pNode4 = CreateListNode(4);
    ListNode* pNode5 = CreateListNode(5);
    ListNode* pNode6 = CreateListNode(6);
    ListNode* pNode7 = CreateListNode(7);

    ConnectListNodes(pNode1, pNode2);
    ConnectListNodes(pNode2, pNode3);
    ConnectListNodes(pNode3, pNode4);
    ConnectListNodes(pNode5, pNode6);
    ConnectListNodes(pNode6, pNode7);

    Test("Test2", pNode1, pNode5, NULL);

    DestroyList(pNode1);
    DestroyList(pNode5);
}

// 1 - 2 - 3 - 4 \
//                7
//         5 - 6 /
void Test3()
{
    ListNode* pNode1 = CreateListNode(1);
    ListNode* pNode2 = CreateListNode(2);
    ListNode* pNode3 = CreateListNode(3);
    ListNode* pNode4 = CreateListNode(4);
    ListNode* pNode5 = CreateListNode(5);
    ListNode* pNode6 = CreateListNode(6);
    ListNode* pNode7 = CreateListNode(7);

    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);
}

// Two lists are same
// 1 - 2 - 3 - 4 - 5
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);

    Test("Test4", pNode1, pNode1, pNode1);

    DestroyList(pNode1);
}

// One of two lists is empty
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);

    Test("Test5", NULL, pNode1, NULL);

    DestroyList(pNode1);
}

// Two lists are empty
void Test6()
{
    Test("Test6", NULL, NULL, NULL);
}

void DestroyNode(ListNode* pNode)
{
    delete pNode;
    pNode = NULL;
}

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

    return 0;
}