两个链表的第一个公共节点

输入两个链表，找出它们的第一个公共的节点。

碰到这种题的时候千万不要用挨个遍历的方法，时间复杂度高

对于两个有相同节点的链表的形状一定是Y。而不是X。然后还可能一个链表长一个链表短，我们可以求出差值，然后让长链表先走差值的长度，然后在挨个比较就可以了。这样时间复杂度就小很多了：

// FirstCommandNodesInLists.cpp : Defines the entry point for the console application.//// 《剑指Offer——名企面试官精讲典型编程题》代码// 著作权所有者：何海涛#include "stdafx.h"#include "..\Utilities\List.h"unsigned int GetListLength(ListNode* pHead);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;        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 - 7void 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 _tmain(int argc, _TCHAR* argv[]){    Test1();    Test2();    Test3();    Test4();    Test5();    Test6();    return 0;}