Leetcode: Linked List Cycle @Python

Use the trick of fast and slow pointers.

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    # @param head, a ListNode
    # @return a boolean
    def hasCycle(self, head):
        if not head: return False
        slow=fast=head
        while fast and fast.next:
            slow=slow.next
            fast=fast.next.next
            if fast==slow:
                return True
        return False