[LeetCode]Linked List Cycle

Question:

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

Follow up:

Can you solve it without using extra space?
Answer:
/**
* 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 *first = head;
ListNode *second = head;
while (second && second -> next)
{
first = first -> next;
second = second -> next -> next;
if (first == second) return true;
}
if (second == NULL || second -> next == NULL) return false;
}
};