142. Linked List Cycle II

這題跟 287. Find the Duplicate Number一樣, 就不再說明了

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