leetCode142:(python)

leetCode142:(python)

解答：

class Solution:
    def detectCycle(self, head):
        if head is None or head.next is None or head.next.next is None:
            return None
        fast = head
        slow = head
        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next
            if fast == slow:
                fast = head
                while fast != slow:
                    slow = slow.next
                    fast = fast.next
                return fast