Why we need to use slow/fast pointer approach while we can just store some unique value of each node? In c++ we can use memory address.

It is simply O(n), because we iterate over all list but we have also O(n) memory for HashSet.

Algorithm is simple. We need to iterate and add each pointer to hashSet until we come across already added one. Then return pointed address.

Code:

ListNode *findCycleStart(ListNode *head) {
    std::unordered_set<ListNode*> visitedNodes;    
    ListNode* it = head;

    do
    {
      visitedNodes.insert(it);
      it = it->next;
    }while(!visitedNodes.contains(it));
    
    return it;
}