Find the radius of convergence and interval of convergence for ∑x^(n!)?

How do I do this? I can't use the ratio test because it's x^n! ... so how would i do that? isn't it supposed to be in the form ∑a_n * x^n ? so then youi'd either have to find lim sup |a_n|^(1/n) or do ratio test ? help!