141. Linked List Cycle

判断一个链表中是否有环存在，用集合set来存储

# 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
        """
        visited = set()
        node = head
        while node is not None:
            if node in visited:
                return True
            else:
                visited.add(node)
                node = node.next
        return False
        

还可以用快慢指针来判断，如果两指针会相交说明有环存在