[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;
    }
};