141. Linked List Cycle【easy】

141. Linked List Cycle【easy】

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

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

 

解法一：

 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 || head->next == NULL) {
13             return false;
14         }
15         
16         ListNode * slow = head;
17         ListNode * fast = head;
18         
19         while (fast->next != NULL && fast->next->next != NULL) {          
20             slow = slow->next;
21             fast = fast->next->next;
22             
23             if (fast == slow) {
24                 return true;
25             }
26         }
27         
28         return false;
29     }
30 };

经典的快慢指针思想