prefix palindrome (prefix function)

ライブラリ 
string rev(string s){
  reverse(ALL(s));
  return s;
}

string prefix_palindrome(const string& s){
  VI pref(s.size()*2 + 10);
  string a = rev(s);
  a = s + "#" + a;
  ll c = 0;
  for (int i = 1; i < (int)a.size(); i++){
    while (c != 0 && a[c] != a[i])
      c = pref[c - 1];
    if (a[c] == a[i])
      c++;
    pref[i] = c;
  }
  return s.substr(0, c);
}

verified