141. Linked List Cycle

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/** * 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) {        ListNode * p = head ;        ListNode * q = head ;        //p->next为何要求不为NULL，因为若p->next为NULL则p->next->next不存在，引发错误        while( p != NULL && q != NULL && p->next != NULL){             p = p->next->next;             q = q->next;             if( p == q){                 return true;             }        }        return false;    }};