算法1

(Hash法)

时间复杂度：O(n)
空间复杂度: O(n)

python3 代码

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def entryNodeOfLoop(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        visited = set()
        p = head
        while p:
            if p in visited:
                return p
            else:
                visited.add(p)
                p = p.next

        return None