3SUM

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;

pair<int,int> twosum(const vector<pair<ll,int>>& s, ll t, ll l, ll r, ll ne1){
    ll ss = s[l].first + s[r].first;

    if(l == ne1)
        return twosum(s, t, l+1, r, ne1);
    else if(r == ne1)
        return twosum(s, t, l, r-1, ne1);
    
    if(l >= r){
        return {-1, -1};
    }
    else if(ss == t){
        return {l, r};
    }
    else if(ss > t){
        return twosum(s, t, l, r-1, ne1);
    }
    else{
        return twosum(s, t, l+1, r, ne1);
    }
}

int main(){
    ull n, x;
    cin >> n >> x;

    vector<pair<ll, int>> as;
    vector<pair<ll, int>> bs;

    as.reserve(n);

    for(ull i=0; i<n; i++){
        ll a;
        cin >> a;
        as.push_back({a,i});
    }

    bs = as;
    sort(bs.begin(), bs.end());

    for(ull i=0; i<n; i++){
        ll tgt = x - bs[i].first;
        auto[a,b] = twosum(bs, tgt, 0, n-1, i);
        if(a == -1){
            continue;
        }

        cout << (bs[a].second+1) << " " << (bs[b].second+1) << " " << (bs[i].second+1) << endl;
        return 0;
    }
    cout << "IMPOSSIBLE" << endl;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED