Trivial implementation of prefix function

trivial_prefix.cpp

std::vector<unsigned> trivial_prefix_function(std::vector<unsigned> & prefix, std::string s) {
    prefix.resize(s.length());
    
    for (int i = 0; i < s.length(); ++i)
        for (int j = 0; j <= i; ++j)
            if (s.substr(0, j) == s.substr(i - j + 1, j))
                prefix[i] = j;
    
    return prefix;
}

int main(int argc, char ** argv) {
    std::string str;
    std::vector<unsigned> res;
    
    std::cin >> str;
    
    trivial_prefix_function(res, str);
    
    for_each(res.begin(), res.end(), [&](unsigned x) {
        cout << x << " ";
    });
    
    return 0;
}