Language: C

#include <bits/stdc++.h>
using namespace std;

template<typename T>
istream& operator >> (istream& istr, vector<T>& v){
    for(T& x: v) istr >> x;
    return istr;
}

template<typename T, typename U>
istream& operator >> (istream& istr, pair<T,U>& p){
    istr >> p.first >> p.second;
    return istr;
}

template<typename T>
ostream& operator << (ostream& ostr, vector<T>& v){
    if(!v.empty()){
        ostr << v.front();
        for(auto itr = ++v.begin(); itr != v.end(); itr++)
            // ostr << *itr;
            ostr << " " << *itr;
    }
    return ostr;
}

template<typename T, typename U>
ostream& operator << (ostream& ostr, pair<T,U>& p){
    // ostr << p.first << p.second;
    // ostr << p.first << ", " << p.second;
    ostr << "(" << p.first << ", " << p.second << ")";
    return ostr;
}

template<typename T>
ostream& operator << (ostream& ostr, vector<vector<T>>& vv){
    if(!vv.empty()){
        ostr << vv.front();
        for(auto itr = ++vv.begin(); itr != vv.end(); itr++)
            ostr << endl << *itr;
    }
    return ostr;
}

template<typename T, typename U>
ostream& operator << (ostream& ostr, vector<pair<T,U>>& vp){
    if(!vp.empty()){
        ostr << vp.front();
        for(auto itr = ++vp.begin(); itr != vp.end(); itr++)
            ostr << endl << *itr;
    }
    return ostr;
}

typedef long long ll;
typedef vector<ll> vll;
typedef vector<char> vc;
typedef vector<double> vd;
// vll v(n); v[i]=x;
typedef vector<vll> vvll;
typedef vector<vc> vvc;
typedef vector<vd> vvd;
// vvll v(m,vll(n)); v[i][j]=x;
typedef pair<ll,ll> pll;
typedef pair<double,double> pd;
typedef vector<pll> vpll;
typedef vector<pd> vpd;
// vpll v(n); v[i]=make_pair(x,y); v[i].first/second;
// v.front() : v[0]
// v.back()  : v[v.size()-1]

#define PI 3.1415926535897932
#define EPS 1e-12
#define INF ((ll)1e+9 + 7)
#define REP(i,n) for(ll i=0; i<(n); i++)
#define RREP(i,n) for(ll i=(n)-1; i>=0; i--)
#define FOR(i,a,b) for(ll i=(a); i<=(b); i++)
#define FORR(i,a,b) for(ll i=(b); i>=(a); i--)
#define fix(x) cout << fixed << setprecision(x)
#define dump(x) cout << #x << " = " << (x) << endl
#define all(x) (x).begin(),(x).end()

template<typename T> T sum(vector<T>& v){return accumulate(all(v), (T)0);}
template<typename T> inline void sort(vector<T>& v){sort(all(v));}
template<typename T> inline void rsort(vector<T>& v){sort(all(v), greater<T>());}
template<typename T> inline void chmin(T& a, T b){if(a>b) a=b;}
template<typename T> inline void chmax(T& a, T b){if(a<b) a=b;}

// n乗(繰り返し二乗法)
ll pow_mod(ll n, ll k, ll mod){
    ll ans = 1;
    for(; k>0; k>>=1){
        if(k&1) ans = (ans * n) % mod;
        n = n * n % mod;
    }
    return ans;
}

// ユーグリッド互除法
ll gcd(ll a, ll b){
    if(b == 0) return a;
    else return gcd(b, a%b);
}

// 拡張ユーグリッド互除法
ll extgcd(ll a, ll b, ll& x, ll& y){
    if(b == 0){
        x = 1;
        y = 0;
        return a;
    }
    ll g = extgcd(b, a%b, y, x);
    y -= a/b*x;
    return g;
}

// 逆元(mod p)
ll modinv(ll a, ll p){
    ll x, y;
    extgcd(a, p, x, y);
    if(x<0) x+=p;
    return x;
}

// 二項係数
#define MAX ((ll)2e5)
ll fac[MAX+1];
ll finv[MAX+1];

void nCr_init(ll max, ll mod){
    fac[0] = 1;
    finv[0] = 1;
    FOR(i, 1, max){
        fac[i] = fac[i-1] * i % mod;
        finv[i] = finv[i-1] * modinv(i, mod) % mod;
    }
}

ll nCr(ll n, ll r, ll mod){
    if(n<r || n<0 || r<0 || r>MAX) return 0;
    ll ans = finv[r];
    if(n>MAX) FORR(i, n-r+1, n) ans = ans * i % mod;
    else ans = (ans * finv[n-r] % mod) * fac[n] % mod;
    return ans;
}

int main(){
    nCr_init(50000, INF);
    const ll MOD = 2019;

    string s;
    cin >> s;

    ll si=0, ri=1, n=s.length(), ans=0;
    vll smod(n);
    REP(i,n){
        si += ((ll)s[n-i-1]-48) * ri;
        si %= MOD;
        smod[n-i-1] = si;
        ri *= 10;
        ri %= 2019;
    }
    // dump(smod);

    map<ll, ll> dmod;
    REP(i,n) dmod[smod[i]]++;
    for(auto& [key, val]: dmod){
        if(key == 0) val++;
        ans += nCr(val, 2, INF);
    }
    cout << ans << endl;
    return 0;
}


Compiler output:
main.c:1:10: fatal error: bits/stdc++.h: No such file or directory
    1 | #include <bits/stdc++.h>
      |          ^~~~~~~~~~~~~~~
compilation terminated.