# 141. Linked List Cycle
## 1.問題
* 判斷linked list是否有cycle
## 2.想法
* 提問
* function header, parameter
* test input
* 觀察
* 說明想法
* 快慢指針: 若是有cycle, 則快慢指針一定會colid
* 測試計算複雜度
## **3.程式碼**
```
/**
* 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* slow = head;
ListNode* fast = head->next;
while (slow && slow->next &&
fast && fast->next && fast->next->next) {
if (slow == fast) {
return true;
}
slow = slow->next;
fast = fast->next->next;
}
return false;
}
};
```
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