Circular List Entry Point

Given the starting node (head) of a singly linked list, determine whether the list contains a cycle (a loop where traversing the list eventually leads back to a previously visited node). If a cycle exists, return the node where the cycle begins — that is, the first node that is part of the cycle when traversing from the head. If no cycle exists in the list, return null.

A cycle exists in a linked list when following the next pointers continuously from the head eventually leads to a node that has already been visited. The cycle entry point is the specific node where the tail of the list connects back, forming the loop.

Important: The linked list must not be modified during or after your algorithm executes. Your solution should only inspect the structure to detect and locate any cycles.

Implementation Note: Internally, a position value pos indicates which node (0-indexed from head) the tail's next pointer connects to. A value of -1 means the tail points to null (no cycle). This pos value is not provided to your function — it is only used to construct the test cases. Your function receives only the head node and must determine the cycle entry point (or absence of cycle) independently.