I like my solution better:

The list node contains a pointer and some data, most likely another pointer, or a primitive type, or a struct of primitive types. Which means, that on all modern systems, a list node is aligned to at least 16 bits, much more likely 32 bits. That means that the pointer to the node always has the last bit set to zero. So:

    bool containsCycle(node_t *root)
    {
        node_t *p = root;
        bool ret = false;
        if (!root) return false; // Empty list

        while (p->next && !ret)
        {
            if ((uintptr_t)p->next & 1)
            {
                ret = true;
                break;
            }
            node_t *q = p->next;
            p->next = (uintptr_t)p->next | 1;
            p = q;
        }
    
        // Reset all pointers
        p = root;
        while (p->next && ((uintptr_t)p->next & 1))
        {
            p->next = (uintptr_t)p->next & ~1;
            p = p->next;
        }
        return ret;
    }

:)