/**
 * 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 == nullptr) {
            return false;
        }
        ListNode * oneStep = head;
        ListNode * twoStep = head;
        while (twoStep ->next != nullptr) {
            oneStep = oneStep->next;
            twoStep = twoStep->next->next;
            if (twoStep == nullptr) {
                break;
            }
            if (oneStep == twoStep) {
                return true;
            }
        }
        return false;
    }
};