LeetCode Linked List Cyle

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Problem Description

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

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

Problem Solution

/**

 * 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 || head->next==NULL)

            return false;

        ListNode *p,*q;

        p=q=head;

        while(p && p->next)

        {

            q=q->next;

            p=p->next->next;

            if(q==p)

                return true;

        }

        return false;

    }

};