142. 环形链表 II

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