Function prefix_function

Strings/Prefix Automaton.cpp:15–25

const int mod = 1e9 + 7;

vector<int> prefix_function(string &s) {
    int n = (int)s.size();
    vector<int> pi(n, 0);
    for (int i = 1; i < n; i++) {
        int j = pi[i-1];
        while (j > 0 && s[i] != s[j]) j = pi[j-1];
        if (s[i] == s[j]) j++;
        pi[i] = j;
    }
    return pi;
}
int aut[N][26];
void compute_automaton(string s)
{