AtCoder Beginner Contest 164 D.Multiple of 2019（DP）

从后往前 $dp$

AC代码：

const int N = 2e5 + 10;
int n, m;
string s;
ll ans, res, now;
int cnt[N];
int main()
{
  cin >> s;
  int len = s.size();
  mem(cnt, 0);
  cnt[0] = 1;
  res = 1;
  ans = 0;
  now = 0;
  per(i, len-1, 0)
  {
    now = (now + ll(s[i] - '0') * res) % 2019;
    ans += cnt[now]++;
    res = res * 10 % 2019;
  }
  pld(ans);
  return 0;
}