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# 141. Linked List Cycle

## 1.問題

* 判斷linked list是否有cycle

![](https://901207480-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LGKoChvN9am4__HCIRK%2F-LNZFcp09zs3Rg5-R4eR%2F-LNZFOdUwJjVVNrJLnji%2F%E8%9E%A2%E5%B9%95%E5%BF%AB%E7%85%A7%202018-09-29%20%E4%B8%8B%E5%8D%883.35.50.png?alt=media\&token=7835b581-ba68-4305-b32e-345d92aa4bbb)

## 2.想法 <a href="#id-2-xiang-fa" id="id-2-xiang-fa"></a>

* 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** <a href="#id-3-cheng-shi" id="id-3-cheng-shi"></a>

```
/**
 * 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;
    }
};
```