linked-list-cycle-ii.py

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

class Solution:
    def detectCycle(self, head: ListNode) -> ListNode:
        if not head or not head.next:
            return None
        
        slow = fast = head
        slow = slow.next
        fast = fast.next.next
        while slow != fast:
            if not fast or not fast.next:
                return None
            slow = slow.next
            fast = fast.next.next
            
        node = head
        while node != slow:
            node = node.next
            slow = slow.next
        
        return node