142. Linked List Cycle II

Medium

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.

class Solution:
    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
        slow = head
        fast = head

        while slow is not None and fast is not None and fast.next is not None:
            slow = slow.next
            fast = fast.next.next

            if slow == fast:
                seek = head
                while True:
                    if seek == slow:
                        return seek

                    seek = seek.next
                    slow = slow.next

        return None


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