LeetCode: Linked List Cycle

## https://code.dennyzhang.com/linked-list-cycle
# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        ## Idea: 2 pointers. Each time, p1 move 1 step, p2 move 2 step. 
        ##         If it has a cycle(not only it's a circle), p1 and p2 will meet. 
        ##         Otherwise p1 and p2 won't.
        ## Complexity: Time O(n), Space O(1)
        ##
        ##     n1    ->   n2    -> n3
        ##     p1
        ##                p2
        if head is None or head.next is None:
            return False
        p1 = head
        p2 = head.next
        while (p1 is not None) and (p2 is not None) and (p2 != p1):
            p1 = p1.next
            if (p2.next is None) or (p2.next is None):
                p2 = None
            else:
                p2 = p2.next.next

        return  (p1 is not None) and (p2 is not None) and (p2 == p1)