先判斷是否是環。假設環長為L,不是環的長度為M，在環中的N處相遇。那麼fast走了M+K1L+N，slow走了M+K2L+N。fast=2slow，M+K1L+N=2*(M+K2*L+N),N=(K1-K2)*L-M。可以看到從N出發再走M就到了環的起始點。

class Solution(object):
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if head == None or head.
 
next ==None:
            return None 
        fast = head 
        slow = head 
        while fast and fast.next:
            
            fast = fast.next.next 
            slow = slow.next 
            if fast == slow:
                break 
        if fast == slow:
            slow = head 
            while
 
 slow != fast:
                slow = slow.next 
                fast = fast.next 
            return slow 
        return None