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

public class Solution {
    public boolean hasCycle(ListNode head) {
        if (head == null) return false;
        ListNode curr = head;
        ListNode runner = head;
        while(runner != null && runner.next != null) {
            head = head.next;
            runner = runner.next.next;
            if (runner == head) return true;
            //if there is cycle, this can end the while loop
            //if there is no cycle, the loop will ends automatically
            //since runner will gets to null
        }
        return false;
    }
}

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """

        if not head:
            return False

        fast = slow = head
        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next
            if fast == slow:
                return True
        return False

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