《剑指offer》第五十二题：两个链表的第一个公共结点
// 面试题52：两个链表的第一个公共结点 // 题目：输入两个链表，找出它们的第一个公共结点。 #include <cstdio> #include "List.h" unsigned int GetListLength(ListNode* pHead); ListNode* FindFirstCommonNode(ListNode* pHead1, ListNode* pHead2) { //获取两个链表长度 unsigned int length1 = GetListLength(pHead1); unsigned int length2 = GetListLength(pHead2); ListNode* pHeadLong = pHead1; ListNode* pHeadShort = pHead2; int lengthDif = length1 - length2; //默认链表1长 if (length1 < length2) //如果链表2更长 { pHeadLong = pHead2; pHeadShort = pHead1; lengthDif = length2 - length1; } //长的链表先走一步 for (int i = 0; i < lengthDif; ++i) pHeadLong = pHeadLong->m_pNext; //然后两个链表一起走找公共节点 while ((pHeadLong != nullptr) && (pHeadShort != nullptr) && (pHeadLong != pHeadShort)) //value和next都相同 { pHeadLong = pHeadLong->m_pNext; pHeadShort = pHeadShort->m_pNext; } return pHeadLong; //第一个公共结点 } unsigned int GetListLength(ListNode* pHead) //遍历链表获得长度 { unsigned int length = 0; ListNode* pNode = pHead; while (pNode != nullptr) { ++length; pNode = pNode->m_pNext; } return length; }
// ====================测试代码==================== void DestroyNode(ListNode* pNode); void Test(char* testName, ListNode* pHead1, ListNode* pHead2, ListNode* pExpected) { if(testName != nullptr) 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, nullptr); 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", nullptr, pNode1, nullptr); DestroyList(pNode1); } // 输入的两个链表有一个空链表 void Test6() { Test("Test6", nullptr, nullptr, nullptr); } void DestroyNode(ListNode* pNode) { delete pNode; pNode = nullptr; } int main(int argc, char* argv[]) { Test1(); Test2(); Test3(); Test4(); Test5(); Test6(); return 0; }
分析：思路很重要。
/* struct ListNode { int val; struct ListNode *next; ListNode(int x) : val(x), next(NULL) { } };*/ class Solution { public: ListNode* FindFirstCommonNode( ListNode* pHead1, ListNode* pHead2) { unsigned int nLength1 = GetListLength(pHead1); unsigned int nLength2 = GetListLength(pHead2); int nLengthDif = nLength1 - nLength2; ListNode* pListHeadLong = pHead1; ListNode* pListHeadShort = pHead2; if (nLength1 < nLength2) { int nLengthDif = nLength2 - nLength1; ListNode* pListHeadLong = pHead2; ListNode* pListHeadShort = pHead1; } for (int i = 0; i < nLengthDif; ++i) pListHeadLong = pListHeadLong->next; while ((pListHeadLong != nullptr) && (pListHeadShort != nullptr) && (pListHeadLong != pListHeadShort)) { pListHeadLong = pListHeadLong->next; pListHeadShort = pListHeadShort->next; } return pListHeadLong; } unsigned int GetListLength(ListNode* pHead) { unsigned int length = 0; ListNode* pNode = pHead; while (pNode != nullptr) { ++length; pNode = pNode->next; } return length; } };
