3SUM

#include <bits/stdc++.h>
using namespace std;

// === Debug macro starts here ===

int recur_depth = 0;
#ifdef DEBUG
#define dbg(x)                                                         \
  {                                                                    \
    ++recur_depth;                                                     \
    auto x_ = x;                                                       \
    --recur_depth;                                                     \
    cerr << string(recur_depth, '\t') << "\e[91m" << __func__ << ":"   \
         << __LINE__ << "\t" << #x << " = " << x_ << "\e[39m" << endl; \
  }
#else
#define dbg(x)
#endif
template <typename Ostream, typename Cont>
typename enable_if<is_same<Ostream, ostream>::value, Ostream&>::type operator<<(
    Ostream& os, const Cont& v) {
  os << "[";
  for (auto& x : v) {
    os << x << ", ";
  }
  return os << "]";
}

// === Debug macro ends here ===

// bit-twiddling
#define bit(x, i) ((x) & (1 << (i)))        // select the bit of position i of x
#define setbit(x, i) ((x) | (1 << (i)))     // set the bit of position i of x
#define unsetbit(x, i) ((x) & ~(1 << (i)))  // etc
#define togglebit(x, i) ((x) ^ (1 << (i)))
#define lowbit(x) ((x) & -(x))  // get the lowest bit of x

// iteration
#define all(x) (x).begin(), (x).end()

// maps, pairs
#define mp make_pair
#define fi first
#define se second

// vectors
#define pb push_back
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<bool> vb;
typedef pair<int, int> ii;

// directions
const int fx[4][2] = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
const int fxx[8][2] = {{0, 1}, {0, -1}, {1, 0},  {-1, 0},
                       {1, 1}, {1, -1}, {-1, 1}, {-1, -1}};

// print pair, vector
template <typename Ostream, typename... Ts>
Ostream& operator<<(Ostream& os, const pair<Ts...>& p) {
  return os << "{" << p.first << ", " << p.second << "}";
}
template <typename T>
ostream& operator<<(ostream& s, vector<T> t) {
  for (const T& v : t) {
    cout << v << " ";
  }
  return s;
}

// types
using ll = long long;
using ull = unsigned long long;
using ui = unsigned int;
using us = unsigned short;

const int INF = 1001001001;
const int MAXN = 10000;

const char br = '\n';

int nxt() {
  int x;
  cin >> x;
  return x;
}

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  // freopen("input.txt", "r", stdin);

  int n = nxt();
  int x = nxt();

  vector<ii> arr(n + 1);
  vb table(5001);
  for (int i = 0; i < n; ++i) {
    int a = nxt();
    table[a] = true;
    arr[i] = mp(a, i + 1);
  }

  sort(all(arr));

  for (int i = 0; i < n - 2; ++i) {
    auto a = arr[i];
    int start = i + 1;
    int end = n - 1;
    while (start < end) {
      auto b = arr[start];
      auto c = arr[end];
      if (a.fi + b.fi + c.fi == x) {
        vi ans;
        ans.pb(a.se);
        ans.pb(b.se);
        ans.pb(c.se);
        sort(all(ans));
        cout << ans << br;
        return 0;
      } else if (a.fi + b.fi + c.fi > x) {
        end--;
      } else {
        start++;
      }
    }
  }
  cout << "IMPOSSIBLE" << br;

  return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: WRONG ANSWER

Test 4

Verdict: ACCEPTED

Test 5

Verdict: WRONG ANSWER

Test 6

Verdict: WRONG ANSWER

Test 7

Verdict: ACCEPTED

Test 8

Verdict: WRONG ANSWER

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: WRONG ANSWER

Test 12

Verdict: RUNTIME ERROR

Test 13

Verdict: RUNTIME ERROR

Test 14

Verdict: RUNTIME ERROR

Test 15

Verdict: RUNTIME ERROR

Test 16

Verdict: RUNTIME ERROR

Test 17

Verdict: RUNTIME ERROR

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: RUNTIME ERROR

Test 21

Verdict: RUNTIME ERROR

Test 22

Verdict: RUNTIME ERROR

Test 23

Verdict: ACCEPTED