LeetCode - 环形链表检测

方法一：哈希表

# 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
        """
        nodesSeen = set([])
        
        while(head!=None):
            if head in nodesSeen:
                return True
            else:
                nodesSeen.add(head)
            head = head.next
        return False

方法二：快慢指针

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        if head == None:
            return False
        
        slow = head
        fast = head.next
        
        while(slow != fast):
            if fast == None or fast.next == None:
                return False
            slow = slow.next
            fast = fast.next.next
        return True