2.6给定一个有环链表，实现一个算法返回环路的开头结点。

LinkedListNode FindBeginning(LinkedListNode head)
{
    LinkedListNode slow = head;
    LinkedListNode fast = head;
    
    //找出碰撞处，会处于链表中LOOP_SIZE - k步的位置
    while (fast != null && fast.next != null)
    {
        slow = slow.next;
        fast = fast.next.next;
        if (slow == fast)
        {
            break;
        }
    }
    
    //错误检查，没有碰撞处，也即没有环路
    if (fast == null || fast.next == null)
    {
        return null;
    }
    
    //slow重新开始走，fast减速接着走
    slow = head;
    while (slow != fast)
    {
        slow = slow.next;
        fast = fast.next;
    }
    
    return fast;
}