这篇博客对上述问题有详细的解释：判断链表中是否有环 ----- 有关单链表中环的问题
这里只做C++的一个代码实现，主要包含构建环形链表，判断是否有环以及环的大小。
#include<iostream> #include<vector> using namespace std; struct Node { int val; Node* next; Node(int num):val(num),next(nullptr){} }; Node* CreatCircularList() { Node* head = new Node(0); Node* ptr = head, *enterNode = nullptr; cout << "Please input a number(0 to entry node, q to quit): "; int temp; bool sign = false; while (cin >> temp) { Node* newNode = new Node(temp); if (!sign&&temp == 0) { enterNode = newNode; sign = true; } newNode->next = ptr->next; ptr->next = newNode; ptr = newNode; cout << "Please input a number(0 to entry node, q to quit): "; } if(sign) ptr->next = enterNode; return head->next; } bool IsLoop(Node* head) { if (head == nullptr || head->next == nullptr) return false; Node* low = head->next; Node* fast = head->next->next; while (fast != nullptr && fast->next != nullptr && fast != low) { fast = fast->next->next; low = low->next; } if (fast == low) return true; else return false; } Node* findEnterVal(Node* head) { if (head == nullptr || head->next == nullptr) return nullptr; Node* low = head->next; Node* fast = head->next->next; while (fast != nullptr && fast->next != nullptr && fast != low) { fast = fast->next->next; low = low->next; } if (fast == low) { low = head; while (low != fast) { low = low->next; fast = fast->next; } return low; } else return nullptr; } int LoopLength(Node* head) { if (!IsLoop(head)) return 0; Node* enterNode = findEnterVal(head); Node* ptr = enterNode->next; int count = 1; while (ptr != enterNode) { count++; ptr = ptr->next; } return count; } int main() { Node* head = CreatCircularList(); cout << "#1 If there is a cycle in it: " << IsLoop(head) << endl; Node* temp = findEnterVal(head); if (temp != nullptr) cout << "#2 Enter node value: " << findEnterVal(head)->val << endl; else cout << "#2 List is not a cycle!" << endl; cout << "#3 Loop length: " << LoopLength(head) << endl; return 0; }