LeetCode: 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?

Two solutions:

1.  o(n) 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) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        unordered_set<ListNode *> visited;
        while (head != NULL) {
            if(visited.find(head) != visited.end()) {
                return true;
            } else {
                visited.insert(head);
            }
            head = head->next;
        }
        return false;
    }
};

2. o(1) Solution: Use two pointers, one steps forward one node at a time, the other steps forward two nodes at a time. If a cycle exists, these two eventually will meet. 

class Solution {

public:
    bool hasCycle(ListNode *head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ListNode *single_forward = head, *double_forward = head;
        
        while (double_forward != NULL) {
            single_forward = single_forward->next;
            double_forward = double_forward->next == NULL ? 
                double_forward->next : double_forward->next->next; 
            if (double_forward != NULL && single_forward == double_forward) {
                return true;
            }
        }
        return false;
    }
};