// https://leetcode.com/problems/linked-list-cycle/
// This is my intial, unthoughtful solution.
// Looking at the answers, it would turn out there's a much better method, involving two pointers, which didn't occur to me when I was originally trying to solve it.
// It seemed impossible to me to "remember" whether we've "seen" a specific node before so as to tell whether a cycle has occurred - the type of reasoning being used in this solution -
// but such reasoning is unnecessary under the other method. Please see 141b for that implementation.

/**
 * Definition for singly-linked list.
 */
#include <cstddef>
#include <unordered_set>
struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
public:
    bool hasCycle(ListNode *head) {
        std::unordered_set<ListNode*> seen;
        ListNode* current = head;
        while (current) {
            if (seen.count(current)) {
                return true;
            }
            seen.insert(current);
            current = current->next;
        }
        return false;
    }
};