Problem p02702 (C++) — buggy submission, verdict: Time Limit Exceeded

/*input
2119
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;

#pragma GCC optimize("unroll-loops,no-stack-protector")
// order_of_key #of elements less than x
//  find_by_order kth element
typedef long long int ll;
#define ld long double
#define pii pair<ll, ll>
typedef tree<pii, null_type, less<pii>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set;
#define f first
#define s second
#define pb push_back
#define REP(i, n) for (ll i = 0; i < n; i++)
#define REP1(i, n) for (int i = 1; i <= n; i++)
#define FILL(n, x) memset(n, x, sizeof(n))
#define ALL(_a) _a.begin(), _a.end()
#define sz(x) (int)x.size()
const ll maxn = 2e5 + 5;
const ll maxlg = __lg(maxn) + 2;
const ll INF64 = 4e18;
const int INF = 0x3f3f3f3f;
const ll MOD = ll(1e9 + 7);
const ld PI = acos(-1);
const ld eps = 1e-9;
#define lowb(x) x & (-x)
#define MNTO(x, y) x = min(x, (__typeof__(x))y)
#define MXTO(x, y) x = max(x, (__typeof__(x))y)
#define SORT_UNIQUE(c)                                                         \\
  (sort(c.begin(), c.end()),                                                   \\
   c.resize(distance(c.begin(), unique(c.begin(), c.end()))))
#define GET_POS(c, x) (lower_bound(c.begin(), c.end(), x) - c.begin())
ll mult(ll a, ll b) { return (a * b) % MOD; }
ll mypow(ll a, ll b, ll mod) {
  if (b <= 0)
    return 1;
  ll res = 1LL;
  while (b) {
    if (b & 1)
      res = (res * a) % mod;
    a = (a * a) % mod;
    b >>= 1;
  }
  return res;
}
int32_t main() {
  ios::sync_with_stdio(false), cin.tie(0);
  string s;
  cin >> s;
  int pref = 0;
  ll ans = 0;
  map<int, int> mp;
  mp[0]++;
  for (int i = sz(s) - 1; i >= 0; i--) {
    pref += (s[i] - '0') * mypow(10, sz(s) - i - 1, 2019);
    pref %= 2019;
    ans += mp[pref];
    mp[pref]++;
  }
  cout << ans;
}


Error: TLE

Fixed submission:

/*input
2119
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;

#pragma GCC optimize("unroll-loops,no-stack-protector")
// order_of_key #of elements less than x
//  find_by_order kth element
typedef long long int ll;
#define ld long double
#define pii pair<ll, ll>
typedef tree<pii, null_type, less<pii>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set;
#define f first
#define s second
#define pb push_back
#define REP(i, n) for (ll i = 0; i < n; i++)
#define REP1(i, n) for (int i = 1; i <= n; i++)
#define FILL(n, x) memset(n, x, sizeof(n))
#define ALL(_a) _a.begin(), _a.end()
#define sz(x) (int)x.size()
const ll maxn = 2e5 + 5;
const ll maxlg = __lg(maxn) + 2;
const ll INF64 = 4e18;
const int INF = 0x3f3f3f3f;
const ll MOD = ll(1e9 + 7);
const ld PI = acos(-1);
const ld eps = 1e-9;
#define lowb(x) x & (-x)
#define MNTO(x, y) x = min(x, (__typeof__(x))y)
#define MXTO(x, y) x = max(x, (__typeof__(x))y)
#define SORT_UNIQUE(c)                                                         \\
  (sort(c.begin(), c.end()),                                                   \\
   c.resize(distance(c.begin(), unique(c.begin(), c.end()))))
#define GET_POS(c, x) (lower_bound(c.begin(), c.end(), x) - c.begin())
ll mult(ll a, ll b) { return (a * b) % MOD; }
ll mypow(ll a, ll b) {
  if (b <= 0)
    return 1;
  ll res = 1LL;
  while (b) {
    if (b & 1)
      res = mult(res, a);
    a = mult(a, a);
    b >>= 1;
  }
  return res;
}
int32_t main() {
  ios::sync_with_stdio(false), cin.tie(0);
  string s;
  cin >> s;
  int pref = 0;
  unordered_map<int, int> mp;
  mp[pref]++;
  ll ans = 0;
  REP(i, sz(s)) {
    unordered_map<int, int> mp2;
    for (auto x : mp) {
      mp2[(x.f * 10) % 2019] += x.s;
    }
    pref = (pref * 10 + s[i] - '0') % 2019;
    // cout<<pref<<' ';
    ans += mp2[pref];
    mp2[pref]++;
    mp = mp2;
  }
  cout << ans;
}