LeetCode 141. Linked List Cycle

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

// Two pointers chasing each other.

    bool hasCycle(ListNode *head) {
        if(!head) return false;
        ListNode* fast = head;
        ListNode* slow = head;
        while(fast && fast->next && fast->next->next) {
            fast = fast->next->next;
            slow = slow->next;
            if(slow == fast) return true;
        }
        return false;
    }

A Cycle means that the address will be visited twice.

   bool hasCycle(ListNode *head) {
        if(!head) return false;
        set<ListNode*> address;
        ListNode* slow = head;
        while(slow) {
            if(address.find(slow) != address.end()) return true;
            address.insert(slow);
            slow = slow->next;
        }
        return false;
    }