CSES – Sum of Four Values

#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,x;
vector<int> v;
set<pair<int,pii>> Lsum, Rsum;
//
signed main(){
    fastio;
    cin>>n>>x;
    for(int i=0;i<n;i++){
        int x; cin>>x;
        v.push_back(x);
    }
    for(int i=0;i<n;i++){
        for(int j=i+1;j<n;j++){
            Rsum.insert({v[i]+v[j],{i,j}});
        }
    }
    for(int i=0;i<n;i++){
        for(int j=i+1;j<n;j++) Rsum.erase({v[i]+v[j], {i,j}});
        for(int j=0;j<i;j++){
            int k = x-(v[i]+v[j]);
            pair<int,pii> p = {k, {-1,-1}};
            auto it = Rsum.lower_bound(p);
            if(it == Rsum.end() || it->f != k) continue;
            cout<<i+1<<" "<<j+1<<" "<<it->s.f+1<<" "<<it->s.s+1<<"\n";
            return 0;
        }
    }
    cout<<"IMPOSSIBLE\n";
    return 0;
}