返回一个存在环路的链表中环路开始的位置

//传入头结点。
+(Node *)findBeginning:(Node *)head{
    Node *slow = head;
    Node *fast = head;
    while (fast != nil && fast.next != nil) {
        slow = slow.next;
        fast = fast.next.next;
        if (slow == fast) {
            break;
        }
    }
    if (fast == nil || fast.next == nil) {
        return nil;
    }
    slow = head;
    while (slow != fast) {
        slow = slow.next;
        fast = fast.next;
    }
    return fast;
}