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问题：

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

快慢指针。

代码：

/** * 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)            return false;        ListNode* slow = head;        ListNode* fast = head->next;        while(fast)        {            if(slow == fast)                return true;            slow = slow->next;            fast = fast->next;            if(fast == NULL)                break;            fast = fast->next;        }        return false;    }};

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