141. Linked List Cycle

problems:

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

Follow up:

Can you solve it without using extra space?

solution:

using namespace std;

 

  //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) {
ListNode* fast = head;
ListNode* slow = head;
while (slow->next != NULL && fast->next->next != NULL)
{
slow = slow->next;
fast = fast->next->next;
if (slow == fast) return true;
}
return false;
}

};