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+# [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;
+    }
+};
+ ```