142. 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):
        ptr=head
        dict1={}
        if ptr==None:
            return None
        while ptr!=None:
            if ptr.next in dict1:
                return ptr.next
            else:
                dict1[ptr]=1
                ptr=ptr.next
        return None