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Linked List Cycle II

Problem Statement​

Given the head of a linked list, return the node where the cycle begins. If there is no cycle, return null.

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 (0-indexed). It is -1 if there is no cycle. Note that pos is not passed as a parameter.

Do not modify the linked list.

Leetcode Link

Example 1:​

ex1

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:​

ex2

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.

Example 3:​

ex3

Input: head = [1], pos = -1
Output: no cycle

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.

Code​

Python Code

class Solution(object):
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if head==None or head.next==None:
            return None
        slow=head.next
        fast=head.next.next
        while fast!=None and fast.next!=None and slow!=fast:
            slow=slow.next
            fast=fast.next.next
        if fast==None or fast.next==None:
            return None
        slow=head
        while slow!=fast:
            slow=slow.next
            fast=fast.next
        return slow