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：

 /**
  * Definition for singly-linked list.
  * struct ListNode {
  *     int val;
  *     ListNode *next;
  *     ListNode(int x) : val(x), next(NULL) {}
  * };
  */
 class Solution {
 public:
     bool hasCycle(ListNode *head) {
         if (head == NULL || head->next == NULL) {
             return false;
         }

         ListNode * slow = head;
         ListNode * fast = head;

         while (fast->next != NULL && fast->next->next != NULL) {
             slow = slow->next;
             fast = fast->next->next;

             if (fast == slow) {
                 return true;
             }
         }

         return false;
     }
 };

经典的快慢指针思想