142. Linked List Cycle II

class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        if(head==NULL ||head->next==NULL){
          return NULL;
        }
        ListNode* slow=head;
        ListNode* fast=head;
         while(fast->next != NULL && fast->next->next != NULL) {
        slow = slow->next;
        fast = fast->next->next;   
        if(slow == fast) {
            fast=head;
            while(slow != fast) {
                slow = slow->next;
                fast = fast->next;
            }
            return slow;
        }
    }
    return NULL;
    }
};