# Question

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

Follow up:

Can you solve it without using extra space?

# Solution

```
# Definition for singly-linked list.
# class ListNode(object):
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution(object):
def hasCycle(self, head):
"""
:type head: ListNode
:rtype: bool
"""
if head is None or head.next is None:
return False
node = head.next
while node is not None:
if node == head:
return True
next_node = node.next
node.next = head
node = next_node
return False
```