102. Linked List Cycle【medium】

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

 

Example

Given -21->10->4->5, tail connects to node index 1, return true

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

解法一：

 class Solution {
 public:
     /**
      * @param head: The first node of linked list.
      * @return: True if it has a cycle, or false
      */
     bool hasCycle(ListNode *head) {
         ListNode * fast, * slow;
         if (head == NULL) {
             return false;
         }
         slow = head;
         fast = head->next;

         while (fast != NULL && fast->next != NULL) {
             if (slow == fast) {
                 return true;
             }
             slow = slow->next;
             fast = fast->next->next;
         }

         return false;
     }
 };