# Linked List Cycle in Python

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#### Beyond Basic Programming - Intermediate Python

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Consider we have a linked list, and we have to check whether there is any cycle or not. To represent the cycle in the given linked list, we will use one integer pointer called pos. This pos represents a position in the linked list where tail is connected. So if pos is -1, then there is no cycle present in the linked list. For example, the linked list is like [5, 3, 2, 0, -4, 7], and pos = 1. So there is a cycle, and tail is connected to the second node.

To solve this, we will follow these steps −

• Take one set as hash set H
• while head is not null −
• if head is present in H, then return true
• return False

## Example

Let us see the following implementation to get better understanding −

class ListNode:
def __init__(self, data, next = None):
self.data = data
self.next = next
def make_list(elements):
for element in elements[1:]:
while ptr.next:
ptr = ptr.next
ptr.next = ListNode(element)
if pos != -1:
p = 0
while p < pos:
ptr = ptr.next
p += 1
return ptr
class Solution(object):
hashS = set()
if head in hashS:
return True
return False
last_node = get_node(head, 5)
pos = 1
last_node.next = get_node(head, pos)
ob1 = Solution()
print(ob1.hasCycle(head))

## Input

List = [5,3,2,0,-4,7]
Pos = 1

## Output

True