pi function

vector<int> pi_function(string P)
{
    std::vector<int> pi = {0};
    for(int i = 1, k = 0; i < P.size(); i++)
    {
        while(k > 0 && P[i] != P[k])
            k = pi[k - 1];
        pi.push_back(k += P[i] == P[k]);
    }
    return pi;
}