linked_list_cycle.cpp

/**
 * 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 || head->next == NULL){
            return false;
        }

        ListNode *slow_ll = head;
        ListNode *fast_ll = head->next;

        while (fast_ll != NULL && fast_ll->next != NULL){
            
            if (slow_ll == fast_ll){
                return true;
            }

            slow_ll = slow_ll->next;
            fast_ll = fast_ll->next->next;
        }

        return false;
    }
};