# [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++
 ```
 /**
 * 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;
    }
};
 ```