ctci-linked-list-cycle.py

"""
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):
    curr = head
    
    seen = set()
    
    while curr:
        if curr in seen:
            return True
        
        seen.add(curr)
        curr = curr.next
    return False