Detect Start of Cycle in Linked List

You are given a singly linked list with $N$ nodes. Each node contains an integer value.\ Your task is to find the **index of the node where a cycle begins**, if a cycle exists. If the linked list has no cycle, print $-1$. A cycle occurs when a node's next pointer points to a **previous node** or **itself** in the list. The $pos$ is the 0-based index of the node that the last node points to. ##