3SUM

input/code.cpp: In function 'std::vector<int> find_summmands(std::vector<long unsigned int>&, uint64_t, int, int)':
input/code.cpp:6:13: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<long unsigned int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    6 |     if (pos == numbers.size()) return {};
      |         ~~~~^~~~~~~~~~~~~~~~~
input/code.cpp:11:25: warning: comparison of integer expressions of different signedness: 'std::vector<int>::size_type' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
   11 |     if (summands.size() == count-1) {
      |         ~~~~~~~~~~~~~~~~^~~~~~~~~~
input/code.cpp:16:25: warning: comparison of integer expressions of different signedness: 'std::vector<int>::size_type' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
   16 |     if (summands.size() == count) {
      |         ~~~~~~~~~~~~~~~~^~~~~~~~

#include <iostream>
#include <vector>
#include <cstdint>

std::vector<int> find_summmands(std::vector<uint64_t> &numbers, uint64_t goal, int count, int pos) {
    if (pos == numbers.size()) return {};
    if (count == 1 && numbers[pos] == goal) return {pos+1};
    if (count == 1 && numbers[pos] != goal) return {};

    auto summands = find_summmands(numbers, goal-numbers[pos], count-1, pos+1);
    if (summands.size() == count-1) {
        summands.push_back(pos+1);
        return summands;
    }
    summands = find_summmands(numbers, goal, count, pos+1);
    if (summands.size() == count) {
        return summands;
    }
    return {};
}

int main() {
    int n;
    uint64_t x;
    std::cin >> n >> x;

    std::vector<uint64_t> numbers(n);
    for (auto &number : numbers) {
        std::cin >> number;
    }

    auto summands = find_summmands(numbers, x, 3, 0);
    if (summands.size() == 0) {
        std::cout << "IMPOSSIBLE" << std::endl;
        return 0;
    }
    for (auto summand : summands) {
        std::cout << summand << " ";
    }
}

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: WRONG ANSWER

Test 12

Verdict: WRONG ANSWER

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: WRONG ANSWER

Test 17

Verdict: WRONG ANSWER

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: WRONG ANSWER

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED