leetcode之Linked List Cycle II

# 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
"""
if not head:
print '1111'
return None
if not head.next:
return None
twostep = head.next.next
distance1 = 0
left = head
#distance1是相遇前一共走了多少步，cycle是环的大小。
while True:
if head.next == twostep:
distance1 = distance1 + 1
cycle = 0
head1 = head.next
while True:
if head1.next != head.next:
cycle = cycle + 1
head1 = head1.next
else:
cycle = cycle + 1
for i in range(distance1 - cycle):
left = left.next
while True:
if left == head.next:
return left
else:
left = left.next
head = head.next
else:
head = head.next
distance1 = distance1 + 1
if not twostep:
return None
elif not twostep.next:
return None
else:
twostep = twostep.next.next