找到环的入口点
当fast若与slow相遇时，slow肯定没有走遍历完链表，而fast已经在环内循环了n圈(1<=n)。假设slow走了s步，则fast走了2s步（fast步数还等于s 加上在环上多转的n圈），设环长为r，则：
2s = s + nr
s= nr
设整个链表长L，入口环与相遇点距离为x，起点到环入口点的距离为a。
a + x = nr
a + x = (n – 1)r +r = (n-1)r + L - a
a = (n-1)r + (L – a – x)
(L – a – x)为相遇点到环入口点的距离，由此可知，从链表头到环入口点等于(n-1)循环内环+相遇点到环入口点，于是我们从链表头、与相遇点分别设一个指针，每次各走一步，两个指针必定相遇，且相遇第一点为环入口点
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def detectCycle(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
if head == None or head.next == None:
return None
slow = fast = head
while fast and fast.next:
slow = slow.next
fast = fast.next.next
if fast == slow:
break
if slow == fast:
slow = head
while slow != fast:
slow = slow.next
fast = fast.next
return slow
return None
