141_Linked_List_Cycle.py

# 2015-10-13  Runtime: 84 ms
# 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
        """
        slow, fast = head, head
        while fast and fast.next:
            slow, fast = slow.next, fast.next.next
            if fast == slow: return True
        return False