class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        ListNode *left = head, *right = head;
        while(right){
            left = left -> next;
            if(!right -> next)
                return nullptr;
            right = right -> next -> next;
            if(left == right){
                ListNode *temp = head;
                while(left != temp){
                    left = left -> next;
                    temp = temp -> next;
                }
                return temp;
            }
        }
        return nullptr;
    }
};

Accepted
16/16 cases passed (8 ms)
Your runtime beats 73.11 % of cpp submissions
Your memory usage beats 98.77 % of cpp submissions (7.3 MB)