# [141. Linked List Cycle](https://leetcode.com/problems/linked-list-cycle/description/) # 思路 判断一个链表是否是环。 设置两个指针p1和p2,用步长分别为1和2从前往后遍历,若链表是环则p1和p2总会相遇。 时间复杂度O(n),空间复杂度O(1) # C++ ``` C++ /** * 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 == NULL || head -> next == NULL) return false; ListNode *p1 = head, *p2 = head -> next; while(p1 && p2 && p2 -> next){ if(p1 == p2) return true; p1 = p1 -> next; p2 = p2 -> next -> next; } return false; } }; ```