141_linked_list_cycle.py

# Solution 1

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

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        node_id = set([])
        ans = True
        while head != None:
            if id(head) not in node_id:
                node_id.add(id(head))
            else:
                break
            head = head.next
        else:
            ans = False
        return ans


# Solution 2

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

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        if head == None:
            return False
        elif head != None and head.next == None:
            return False
        else:
            slow = head
            fast = head.next
            ans = False
            while slow != fast:
                if (fast.next == None) or (fast.next.next == None):
                    break
                slow = slow.next
                fast = fast.next.next
            else:
                ans = True
            return ans

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