力扣142. 环形链表 II

/**
 * 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) {
        ListNode* slow = head;
        ListNode* fast = head;
        while(fast && fast->next) {
            fast = fast->next->next;
            slow = slow->next;
            if(fast == slow) {
                break;
            }
        }
        if(fast == nullptr || fast->next == nullptr) return NULL;
        ListNode* idx1 = head;
        ListNode* idx2 = fast;
        while(idx1 != idx2) {
            idx1 = idx1->next;
            idx2 = idx2->next;
        }
        return idx1;
    }
};