## Solutions
### slow and fast pointers
```python
# Definition for singly-linked list.
# class ListNode(object):
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution(object):
def middleNode(self, head):
"""
:type head: Optional[ListNode]
:rtype: Optional[ListNode]
"""
fast = head
slow = head
while fast and fast.next:
slow = slow.next
fast = fast.next.next
return slow
```
#### Complexity Analysis
- Time complexity is On, because we use a while loop
- Space complexity is O1, because we only declare 2 pointers