## Solutions
### slow and fast pointers
```python
# 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
"""
slow = fast = head
while fast and fast.next:
fast = fast.next.next
slow = slow.next
if fast == slow:
return True
return False
```
#### Complexity Analysis
- Time complexity is On, because we use a while loop
- Space complexity is O1, because we only declare 2 pointers