3SUM

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
typedef long long ll;

int main() {
    const int MAXN = 5000;
    int n, x;
    cin >> n >> x;

    ll nums[MAXN] = {0};
    ll origNums[MAXN] = {0};
    for (int i=0; i<n; i++) {
        cin >> nums[i];
        origNums[i] = nums[i];
    }
    sort(nums, nums + n);

    int startInd = 0;
    int endInd = n-1;

    while (startInd < endInd) {
        ll rem = x - nums[startInd] - nums[endInd];
        if (rem <= 0) {
            endInd--;
            continue;
        }
        int ind3 = lower_bound(nums, nums + n, rem) - nums;
        if (ind3 >= n || nums[ind3] != rem) {
            startInd++;
            continue;
        }

        if (nums[ind3] == rem) {
            for (int i=0; i<n; i++) {
                if (origNums[i] == nums[startInd]) {
                    cout << i+1;
                }
            }
            for (int i=0; i<n; i++) {
                if (origNums[i] == nums[endInd]) {
                    cout << " " << i+1;
                }
            }
            for (int i=0; i<n; i++) {
                if (origNums[i] == nums[ind3]) {
                    cout << " " << i+1 << "\n";
                }
            }
            return 0;
        }
    }

    cout << "IMPOSSIBLE\n";

    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: WRONG ANSWER

Test 3

Verdict: ACCEPTED

Test 4

Verdict: WRONG ANSWER

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: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: WRONG ANSWER

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: WRONG ANSWER