# 141.Linked List Cycle

**141.Linked List Cycle**

难度:Easy

> 给定一个链表，判断链表中是否有环。 为了表示给定链表中的环，我们使用整数 pos 来表示链表尾连接到链表中的位置（索引从 0 开始）。 如果 pos 是 -1，则在该链表中没有环。

```
示例 1：

输入：head = [3,2,0,-4], pos = 1
输出：true
解释：链表中有一个环，其尾部连接到第二个节点。
3->2->0->-4->2->...
示例 2：

输入：head = [1,2], pos = 0
输出：true
解释：链表中有一个环，其尾部连接到第一个节点。
1->2->1->...

示例 3：

输入：head = [1], pos = -1
输出：false
解释：链表中没有环。
1
```

进阶： 你能用 O(1)（即，常量）内存解决此问题吗？

方法：快慢指针。

```
/**
 * 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 || !head->next || !head->next->next) return false;
        ListNode* slow=head->next;
        ListNode* fast=head->next->next;
        
        while(fast->next)
        {
            if(slow == fast) return true;
            slow=slow->next;
            fast=fast->next->next;    
            if(!slow || !fast) break;
            //cout<<slow->val<<fast->val<<endl;
        }
        return false;
    }
};
```


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