linked_list_cycle.cpp

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

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list 
where tail connects to. If pos is -1, then there is no cycle in the linked list.

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.
*/

/**
 * 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)
            return false;
        struct ListNode *slow=head;
        struct ListNode *fast=head->next;
        while(slow!=fast)
        {
            
            if(fast==NULL || fast->next==NULL)//reached end of linked list
                return false;
            slow=slow->next;
            fast=fast->next->next;
        }
        return true;
    }
};