Design an NFA to accept a, ad, abc, abd, but not e, abcd?

Question

I've made the following diagram to accept a, ad, abc, abd but I don't it to accept e, abcd, how can I limit that?

Updated I modified the diagram in another attempt:

Answer 1

Your automaton has loops so it accepts an infinite language.

For finite languages $\{w_1, \dots, w_n\}$, it's easy to write them as regular expression

$\qquad w_1 \mid \ldots \mid w_k$

and apply Thompson's construction. If the resulting automaton is too ugly for your taste, determinize and minimize according to the canonical textbooks.

Warning: Determinizing automata for finite languages can blow up automaton size exponentially.

Note how this approach scales neatly to more concise representations of finite languages and even infinite languages -- all you need is a regular expression. Or any other formalism equivalent to finite automata, as the proofs of equivalence are usually constructive.

Answer 2

Just make an automaton that looks like a tree, for each state splitting into the possible letters.

In data structure terminology such a tree with leters on edges is called a trie, as in re-trie-val.

Answer 3

This is the correct answer for the specified specification: