LinkedList Cycle starting point

Given a linked list. Return pointer to the node where cycle starts. If there is no cycle, return null.

Definition of a single linked list:

// Java
class ListNode {
  int val;
  ListNode next;
  ListNode(int x) {
    val = x;
    next = null;
  }
}

// C++
class ListNode
{
public:
  int val;
  ListNode *next;
  ListNode(int x) : val(x), next(NULL) {}
};

// C
struct ListNode
{
  int val;
  struct ListNode *next;
};

Implement a function detectCycle that returns a pointer to the node where cycle starts. If there is no cycle, return null.

// Java
ListNode detectCycle(ListNode head)

// C, C++
ListNode* detectCycle(ListNode *head)

Example

Function detectCycle returns null because this linked list does not contain a cycle.

Function detectCycle returns pointer to the node with value 3 - the starting point of the cycle.