单链表是否有环及环入口点
参考与原理：https://www.jianshu.com/p/ef71e04241e4
https://blog.csdn.net/danny_idea/article/details/89504629
1 #include <bits/stdc++.h> 2 using namespace std; 3 4 typedef struct node pnode; 5 struct node 6 { 7 int info; 8 pnode *next; 9 }; 10 11 void create(pnode **head) //创建链表 12 { 13 (*head) = (pnode *)malloc(sizeof(pnode)); 14 (*head)->info = 0; //创建头节点，记录链表节点数 15 (*head)->next = nullptr; 16 } 17 18 void add(pnode *head, int val) //尾插法给链表添加节点 19 { 20 head->info++; 21 22 while (head->next != nullptr) 23 head = head->next; 24 25 pnode *temp = (pnode *)malloc(sizeof(pnode)); 26 temp->info = val; 27 temp->next = nullptr; 28 head->next = temp; 29 } 30 31 void show(pnode *head) //打印链表 32 { 33 printf("[%d] ", head->info); 34 head = head->next; 35 36 while (head != nullptr) 37 { 38 printf("%d ", head->info); 39 head = head->next; 40 } 41 } 42 43 void makeloop(pnode *head, int val) //给链表建环，val为环入口，下标从1开始 44 { 45 pnode *temp = head; 46 47 while (val--) 48 temp = temp->next; 49 50 while (head->next != nullptr) 51 head = head->next; 52 53 head->next = temp; 54 } 55 56 void judgeloop(pnode *head) //判断链表是否有环 57 { 58 pnode *fast, *slow; 59 60 fast = slow = head; 61 62 do 63 { 64 fast = fast->next; 65 if (fast != nullptr) //这里要记得判断一次是否为空 66 fast = fast->next; 67 68 slow = slow->next; 69 } while (fast!=nullptr && fast!=slow); 70 71 if (fast == slow) 72 printf("有环\n"); 73 74 else 75 printf("无环\n"); 76 } 77 78 void findEntrance(pnode *head) //找链表环入口，下标从1开始 79 { 80 pnode *fast, *slow; 81 82 fast = slow = head; 83 84 do 85 { 86 fast = fast->next; 87 if (fast != nullptr) 88 fast = fast->next; 89 90 slow = slow->next; 91 } while (fast!=nullptr && fast!=slow); //先使两指针相遇 92 93 slow = head; 94 int ind = 0; //记录下标 95 96 while (fast != slow) 97 { 98 slow = slow->next; 99 fast = fast->next; 100 ind++; 101 } 102 103 printf("入口节点下标：%d\n", ind); 104 } 105 106 int main() 107 { 108 pnode *head; 109 110 create(&head); 111 add(head, 1); 112 add(head, 2); 113 add(head, 3); 114 add(head, 4); 115 add(head, 5); 116 add(head, 6); 117 show(head); 118 judgeloop(head); 119 makeloop(head, 3); 120 judgeloop(head); 121 findEntrance(head); 122 123 return 0; 124 }
