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     //1、確定環中結點數目n
12     //2、兩個指針  1個先走n步，然後兩個一起走
13     //3、相遇點即為入口點
14     ListNode *detectCycle(ListNode *head) {
15         if(head == NULL)
 
16             return NULL;
17         ListNode *node = MettingNode(head);
18         if(node == NULL)//無環
19         {
20             return NULL;
21         }
22         int nodeNum = 1;
23         ListNode *pNode = node;
24         while(pNode->next != node)
25         {
26             pNode =pNode->next;
 
27             nodeNum++;
28         }
29         pNode = head;//pNode指向頭結點
30         for(int i=0;i<nodeNum;i++)
31         {
32             pNode = pNode->next;
33         }
34         ListNode *pNode2  = head;
35         while(pNode2 != pNode)//兩結點不相遇
36         {
37             pNode = pNode->next;
38             pNode2 = pNode2->next;
39         }
40         return pNode;
41     }
42     //快慢指針找環中相遇結點
43     //找到相遇節點就可確定環中結點數目
44     ListNode *MettingNode(ListNode *head)
45     {
46         if(head == NULL)
47             return NULL;
48         ListNode *slow = head;
49         ListNode *fast = head;
50         while(fast != NULL && fast->next != NULL)
51         {
52             slow = slow->next;
53             fast = fast->next->next;
54             if(slow == fast)
55                 return slow;
56         }
57         return NULL;
58     }
59 };

leetcode鏈表--6、linked-list-cycle-ii(有環單鏈表環的入口結點)