快慢指针
/** * 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) return false; ListNode *fast=head; ListNode *slow=head; while(fast!=NULL && fast->next!=NULL) { fast = fast->next->next; slow = slow->next; if(fast==slow) return true; } return false; } };