LeetCode - Linked List Cycle

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

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

/** * 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 *p=head,*q=head;        while(p!=NULL&&q!=NULL){            p=p->next;            q=q->next;                        if(q!=NULL){                q=q->next;            }else{                return false;            }                        if(p==q){                return true;            }        }        return false;    }}; 

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