142. 环形链表 II - 力扣（LeetCode）

/**
 * 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==NULL){
            return NULL;
        }
        ListNode* fast=head;
        ListNode* slow=head;

        while(true){
            if(fast->next==NULL||fast->next->next==NULL){
               return NULL;
            }
            fast=fast->next->next;
            slow=slow->next;
            if(slow==fast){
                break;
            }
        }

        fast=head;
        while(true){
            if(fast==slow){
                return fast;
            }
            fast=fast->next;
            slow=slow->next;
            
        }
        
    }
};