<filename>Day_1_linkedList/linkedList_cycle_2.py
'''
Leet code link: https://leetcode.com/problems/linked-list-cycle-ii/
Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Note: Do not modify the linked list.

Example 1:

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

Example 2:

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

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

# Definition for singly-linked list.


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


class Solution(object):
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        current = head

        while current is not None:
            if hasattr(current, 'visited'):

                return current

            current.visited = True
            current = current.next
        return None
