Pushdown automaton
In computer science, in particular automata theory, pushdown automata (PDA) are abstract devices that recognize context-free languages.
Informally, a pushdown automaton is a finite automaton outfitted with access to a potentially unlimited amount of memory in the form of a single stack. The finite automaton in question is usually nondeterministic (resulting in a nondeterministic pushdown automaton or NPDA) since deterministic pushdown automata cannot recognize all context-free languages.
If we allow a finite automaton access to two stacks instead of just one, we obtain a more powerful device - equivalent in power to a Turing machine. A Stack machine
