141 Linked List Cycle

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

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail’s next pointer is connected to. Note that pos is not passed as a parameter.

Return true if there is a cycle in the linked list. Otherwise, return false.

Example 1:

[3] -> [2] -> [0] -> [4]
        ^_____________|

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).

Example 2:

[1] -> [2]
 ^______|

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.

Example 3:

[1]

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

Constraints:

  • The number of the nodes in the list is in the range [0, 104].
  • -105 <= Node.val <= 105
  • pos is -1 or a valid index in the linked-list.

Follow up: Can you solve it using O(1) (i.e. constant) memory?

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

''' Set '''
class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        seen = set()
        while head != None:
            if head not in seen:
                seen.add(head)
                head = head.next
            else:
                return True
        return False

''' Two pointers '''
class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        p1 = p2 = head
        while p2 is not None and p2.next is not None:
            p1 = p1.next
            p2 = p2.next.next
            if p1 == p2:
                return True
        return False