142. Linked List Cycle II

class Solution:
    def detectCycle(self, head):
        if not head or not head.next:
            return None
        slow = fast = head
        # Phase 1: detect cycle
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
            if slow == fast:
                # Phase 2: find entry
                ptr = head
                while ptr != slow:
                    ptr = ptr.next
                    slow = slow.next
                return ptr
        return None