CSES – Subarray Divisibility

#include<bits/stdc++.h>
#define int long long
#define pii pair<int,int>
#define f first
#define s second
#define fastio ios_base::sync_with_stdio(0);cin.tie(0)
using namespace std;
//declare
int n;
vector<int> v;
//
int mod(int x){
    x %= n;
    if(x < 0) x += n;
    return x;
}
signed main(){
    fastio;
    cin>>n;
    for(int i=0;i<n;i++){
        int x; cin>>x;
        v.push_back(x);
    }
    vector<int> pre;
    pre.push_back(0);
    for(int i:v) pre.push_back(mod(pre.back() + i));
    sort(pre.begin(),pre.end());
    int ans = 0;
    for(int i=0;i<n;i++){
        int x = upper_bound(pre.begin()+i+1,pre.end(),pre[i])
               - lower_bound(pre.begin()+i+1,pre.end(),pre[i]);
        ans += x;
    }
    cout<<ans<<"\n";
    return 0;
}