JZ-C-37

剑指offer第三十七题：两个链表的第一个公共结点

//============================================================================
// Name        : JZ-C-37.cpp
// Author      : Laughing_Lz
// Version     :
// Copyright   : All Right Reserved
// Description : 两个链表的第一个公共结点
//============================================================================

#include <iostream>
#include <stdio.h>
#include "List.h"
using namespace std;

unsigned int GetListLength(ListNode* pHead);
/**
 * 先移动长链表指针至某位置（与短链表持平），再同时遍历，遇到第一个相同的结点即为公共结点
 * 时间复杂度为O(m+n)
 */
ListNode* FindFirstCommonNode(ListNode *pHead1, ListNode *pHead2) {
    // 得到两个链表的长度
    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;
    }

    // 先在长链表上走几步，再同时在两个链表上遍历
    for (int i = 0; i < nLengthDif; ++i)
        pListHeadLong = pListHeadLong->m_pNext;

    while ((pListHeadLong != NULL) && (pListHeadShort != NULL)
            && (pListHeadLong != pListHeadShort)) {
        pListHeadLong = pListHeadLong->m_pNext;//当不存在公共结点时，返回NULL
        pListHeadShort = pListHeadShort->m_pNext;
    }

    // 得到第一个公共结点
    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;
}

// ====================测试代码====================
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");
}

// 第一个公共结点在链表中间
// 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);
}

// 没有公共结点
// 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);
}

// 公共结点是最后一个结点
// 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);
}

// 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(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);
}

// 输入的两个链表有一个空链表
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);
}

// 输入的两个链表有一个空链表
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;
}