/**
* 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) {
if(head == nullptr)
return nullptr;
ListNode* f = head;
ListNode* s = head;
while(f->next!=nullptr && f->next->next!=nullptr){
f = f->next->next;
s = s->next;
if(f == s){
f=head;
break;
}
}
if(f->next == nullptr || f->next->next == nullptr)
return nullptr;
while(f != s){
f = f->next;
s = s->next;
}
return s;
}
};