leetcode 141 Linked List Cycle

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

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

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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) {
        if(head == NULL) return false;
		ListNode *next = head->next;
        
		while(next!=NULL) {
			if(next == head) return true;
			head = head->next;
			next = next->next;
			if(next!=NULL)
			    next = next->next;
		}

		return false;
    }
};