【每日一题leetcode】-141 Linked List Cycle

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

由于最开始没有弄清楚链表中循环可以从任何位置开始，判断条件是是否tail的值等于head。但是这个题是判断一个链表中是否存在有环，因此这个循环的起始位置是未知的。所以改变了思路，采用两个指针，一个每次走一步，一个每次走两步。如果有回路存在，那么某个时刻两个指针相同。

Runtime: 48 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
        """
        
        if head == None:
            return False
        else:
            slow = head
            fast = head
        while fast.next!= None and fast.next.next != None:    
            slow = slow.next
            fast = fast.next.next
            if slow == fast:
                return True       
           
        
        
        return False