dfa.cpp

#include <iostream>
#include <vector>

using namespace std;

int main()
{
    vector<vector<int>> dfa
        {
            //      a  b
            /*0*/ { 1, 0 },
            /*1*/ { 1, 2 },
            /*2*/ { 3, 0 },
            /*3*/ { 1, 2 },
        };
    int state = 0;
    char c;
    while (cin >> c) {
        state = dfa[state][c - 'a'];
    }
    cout << "Input is " << (state == 3 ? "" : "not ") << "in the language\n";
}