Linked List Cycle 判斷一個鏈表是否存在迴路(循環)

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

注意，鏈表循環並非尾指針和頭指針相同，多是在中間某一段造成一個環路，因此不能只判斷元素和第一個元素是否存在重合

先設置兩個指針p_fast和p_slow。從頭開始遍歷鏈表，p_fast每次走兩個節點，而p_slow每次走一個節點，若存在循環，這兩個指針一定重合：

 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     bool hasCycle(ListNode *head) {
12         if(head == NULL) return false;
13         
14         ListNode *p_fast = head;
15         ListNode *p_slow = head;
16         
17         do{
18             p_slow = p_slow->next;
19             if(p_fast != NULL)
20                 p_fast = p_fast->next;
21             if(p_fast != NULL)
22                 p_fast = p_fast->next;
23             else
24                 return false;
25         }while(p_fast != p_slow);
26         
27         return true;
28         
29     }
30 };