Linked List Cycle II

LeetCode 142

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

class Solution(object):
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        slow = fast = head
        
        while fast and fast.next and fast.next.next:
            slow = slow.next
            fast = fast.next.next
            # 利用 is 
            if slow is fast:
                slow = head
                while slow is not fast:
                    slow = slow.next
                    fast = fast.next
                return slow
        return