Cycle Detection:

"""
Detect a cycle in a linked list. Note that the head pointer may be 'None' if the list is empty.

A Node is defined as: 

    class Node(object):
        def __init__(self, data = None, next_node = None):
            self.data = data
            self.next = next_node
"""

def has_cycle(head):
    l=[]
    while head!=None:
        if head.data not in l:
            l.append(head.data)
            head = head.next
        else:
            return True