3SUM

#include <algorithm>
#include <cstdio>
#include <iostream>
#include <utility>
#include <vector>
#include <bits/stdc++.h>

using namespace std;
using ll = long long;

// Debug printing
#ifdef DEBUG
#define deb(fmt, args...) printf("DEBUG: %d: " fmt, __LINE__, ##args)
#else
#define deb(fmt, args...)
#endif

void print_array(vector<int> in, const string title = "Vector")
{
	cout << title << " [\n";
	for (const auto& el : in) {
		cout << el << " ";
	}
	cout << "\n] END\n";
}

void print_matrix(vector<vector<int>> in, const string title = "Matrix")
{
	cout << title << "[\n";
	for (unsigned int i = 0; i < in.size(); i++) {
		for (unsigned int j = 0; j < in[i].size(); j++) {
			cout << in[i][j] << " ";
		}
		cout << "\n";
	}
	cout << "] END\n";
}

int main(int argc, char *argv[])
{
	// Read the input parameters
	int n, t;
	cin >> n >> t;

	// Read values from one line
    // vector<vector<int>> A(n, vector<int>(t));
	vector<pair<int, int>> in(n);
    int tmp;
	for (int i = 0; i < n; i++) {
		cin >> tmp;
        in[i] = {tmp, i + 1};
	}

    sort(in.begin(), in.end());

    for (int i = 0; i < n - 2; i++) {
        // skip duplicates
        if (i > 0 && in[i] == in[i - 1]) continue;

        int l = i + 1;
        int r = n - 1;
        int sum = t - in[i].first;
        int s;

        while (l < r) {
            s = in[l].first + in[r].first;
            if (s == sum) {
                printf("%d %d %d\n", in[i].second, in[l].second, in[r].second);
                return 0;
            }
            if (s > sum) {
                r--;
            } else {
                l++;
            }
        } 
    }

    cout << "IMPOSSIBLE\n";

	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: 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