AtCoder Beginner Contest 378 E

https://atcoder.jp/contests/abc378/tasks/abc378_e
https://atcoder.jp/contests/abc378/editorial/11300

#include<bits/stdc++.h>
#define x first
#define y second
#define all(x) (x).begin(),(x).end()
#define lowbit(x) (x)&-(x)
using namespace std;
typedef long long ll;
typedef pair<ll,ll> pii;
const int N=2e5+10,mod=1e9+7;
ll n,m,k;
ll tree[N],a[N],s[N];
void update(int p){
    p++;
    while(p<=m){
        tree[p]++;
        p+=lowbit(p);
    }
}
ll query(int p){
    ll res=0;
    p++;
    while(p){
        res+=tree[p];
        p-=lowbit(p);
    }
    return res;
}
void solve(){
    cin>>n>>m;
    ll sum=0,res=0;
    //update(0);
    for(int i=1;i<=n;i++){
        cin>>a[i];
        s[i]=(s[i-1]+a[i])%m;
        res+=s[i]*i-sum;
        sum+=s[i];
        res+=m*(i-1-query(s[i]));
        update(s[i]);
    }
    cout<<res<<endl;
}
int main(){
    ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);

    int _=1;
    //cin>>_;
    while(_--) solve();

    return 0;
}