3SUM

#include <iostream>
#include <vector>
#include <algorithm>


int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(NULL);
    int n, x;
    std::cin >> n >> x;
    std::vector<int> array(1, 0);
    for (int i = 1; i <= n; i++) {
        array.push_back({i});
    }
    if (n < 3) {
        std::cout << "IMPOSSIBLE\n";
        return EXIT_FAILURE;
    }
    int min = *std::min_element(array.begin() + 1, array.end());
    int first = 1;
    int second = first + 1;
    int third = n;
    bool b = (array[first] + array[second] + array[third] != x);
    bool turn  = true;
    while (b) {
        if (first >= second || second >= third) break;
        if (array[first] + min >= x) {first++; continue;}
        if (array[third] + min >= x) {third--; continue;}
        while (second < third && b) {
            b = (array[first] + array[second] + array[third] != x);
            second += b;
        }
        if (!b) break;
        first += turn;
        third += 1 - turn;
        turn = !turn;
    }
    if (b) std::cout << "IMPOSSIBLE\n";
    else {
        std::cout << first << ' ' << second << ' ' << third << std::endl;
    }
}

Test 1

Verdict: RUNTIME ERROR

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: WRONG ANSWER

Test 6

Verdict: WRONG ANSWER

Test 7

Verdict: WRONG ANSWER

Test 8

Verdict: WRONG ANSWER

Test 9

Verdict: WRONG ANSWER

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: WRONG ANSWER

Test 19

Verdict: ACCEPTED

Test 20

Verdict: WRONG ANSWER

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: RUNTIME ERROR