LetCode Hot100 142.环形链表II

class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        int flag = 0;
        ListNode *slow = head, *fast = head;
        while(fast && fast->next){
            slow = slow->next;
            fast = fast->next->next;
            if(slow == fast){
                //首次相遇点
                flag = 1;
                break;
            }
        }
        if(flag == 1){
            ListNode* curNode = head;
            while(curNode != slow){
                curNode = curNode->next;
                slow = slow->next;
            }
            return curNode;
        }        
        return nullptr;
    }
};