3SUM

input/code.cpp: In function 'void Test()':
input/code.cpp:24:12: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   24 |     freopen("temp\\in.txt", "r", stdin);
      |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~

#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N=5005;

int a[N];
struct Node
{
    int value;
    int p1, p2;

    Node(int v_, int p1_, int p2_):
        value(v_), p1(p1_), p2(p2_) {}

    bool operator < (const Node& node_)const {
        return value < node_.value;
    }
};
vector<Node> nodes;

set<Node> s;
void Test()
{
    freopen("temp\\in.txt", "r", stdin);
}
int main()
{
    // Test();
    int n, x;
    cin >> n >> x;

    for(int i=1; i<=n; i++)
        cin >> a[i];
    for(int i=1; i<=n; i++)
        for(int j=i+1; j<=n; j++)
            nodes.push_back(Node(a[i] + a[j], i, j));
    sort(nodes.begin(), nodes.end());

    for(int i=1; i<=n; i++)
    {
        // check x - a[i]
        int left = x - a[i];
        auto it = lower_bound(nodes.begin(), nodes.end(), Node(left, 0, 0));
        if(it == nodes.end())
            continue;
        auto node = *it;
        if(i != node.p1 && i != node.p2 && left == node.value)
        {
            cout << i << " " << node.p1 << " " << node.p2;
            return 0;
        }
    }
    cout << "IMPOSSIBLE";

    return 0;
}

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: TIME LIMIT EXCEEDED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: TIME LIMIT EXCEEDED

Test 17

Verdict: TIME LIMIT EXCEEDED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: TIME LIMIT EXCEEDED

Test 21

Verdict: TIME LIMIT EXCEEDED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED