Double pointer circular linked list

A daily topic ing, Today is a day medium topic 142. Linked List Cycle II

/** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode(int x) : val(x), next(NULL) {} * }; */
class Solution {
    
public:
    ListNode *detectCycle(ListNode *head) {
    
        ListNode *fast=head,*slow=head;
        do{
    
            if(!fast||!fast->next)
                return nullptr;
            slow=slow->next;
            fast=fast->next->next;
        }while(fast!=slow);
        fast=head;
        while(fast!=slow){
    
            fast=fast->next;
            slow=slow->next;
        }
        return fast;
    }
};