/** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode(int x) : val(x), next(NULL) {} * }; */ class Solution { public: bool hasCycle(ListNode *head) { if (head == nullptr) { return false; } ListNode * oneStep = head; ListNode * twoStep = head; while (twoStep ->next != nullptr) { oneStep = oneStep->next; twoStep = twoStep->next->next; if (twoStep == nullptr) { break; } if (oneStep == twoStep) { return true; } } return false; } };