Linked_List_Cycle.py

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

Follow up:
Can you solve it without using extra space?
"""

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    # @param head, a ListNode
    # @return a boolean
    def hasCycle(self, head):
        slow = head
        fast = head
        while fast is not None and fast.next is not None:
            slow = slow.next
            fast = fast.next.next
            if fast == slow:
                return True
        return False