Leetcode-Easy 141. Linked List Cycle

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:  
    def hasCycle(self, head):  
        # write your code here  
        if head is None:  
            return False  
        if head.next is None:  
            return False  
        slow = head.next  
        fast = head.next  
        if fast.next is not None:  
            fast = fast.next  
        while (slow.next is not None) and (fast.next is not None):  
            if slow == fast:  
                return True  
            slow = slow.next  
            fast = fast.next  
            if fast.next != None:  
                fast = fast.next  
        return False