(142)Linked List Cycle II

class Solution {
public:
    ListNode *detectCycle(ListNode *head) 
    {
        if(!head || !head->next) return nullptr;
        ListNode *sptr = head;
        ListNode *fptr = head->next;

        while(fptr) {
            if(fptr <= sptr) return fptr;
            sptr = sptr->next;
            fptr = fptr->next;
        }
        return nullptr;    
    }
};