leetcode+判断链表是否有环，双指针

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struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};
class Solution {
    public:
    bool hasCycle(ListNode *head) {
        ListNode* first =head;
        ListNode* second =head;
        while (first!=NULL && second!=NULL && second->next!=NULL) {
            first=first->next;
            second = second->next->next;
            if(first==second) return true;
        }
        return false;
    }
};
int main()
{
    
    return 0;
}