题解 | #判断链表中是否有环#

头部指针的表达

在这道题中，Head指针用的也是ListNode的结构。
这里用到快慢指针。

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

#
# 
# @param head ListNode类 
# @return bool布尔型
#
class Solution:
    def hasCycle(self , head ):
        # write code here
        if head is None or head.next is None:
            return False
        walker = head
        runner = head.next
        while runner is not None and walker != runner:
            walker = walker.next
            runner = runner.next
            if runner!=None:
                runner = runner.next
        if runner==walker:
            return True
        return False