LeetCode – Refresh – Linked List Cycle II

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode *detectCycle(ListNode *head) {
12         if (!head || !head->next) return NULL;
13         ListNode *slow = head, *fast = head;
14         bool flag = false;
15         while (fast && fast->next) {
16             slow = slow->next;
17             fast = fast->next->next;
18             if (slow == fast) {
19                 flag = true;
20                 break;
21             }
22         }
23         if (!flag) return NULL;
24         fast = head;
25         while (slow != fast) {
26             slow = slow->next;
27             fast = fast->next;
28         }
29         return slow;
30     }
31 };