3SUM

input/code.cpp: In function 'int main()':
input/code.cpp:23:40: warning: comparison of integer expressions of different signedness: 'size_t' {aka 'long unsigned int'} and 'll' {aka 'long long int'} [-Wsign-compare]
   23 | #define fr(i,a,b) for (size_t i = a; i < b; i++)
      |                                        ^
input/code.cpp:49:3: note: in expansion of macro 'fr'
   49 |   fr(i, 0, n) {
      |   ^~
input/code.cpp: In instantiation of 'main()::<lambda(auto:48, auto:49)> [with auto:48 = long long int; auto:49 = long unsigned int]':
input/code.cpp:85:19:   required from here
input/code.cpp:67:23: warning: comparison of integer expressions of different signedness: 'll' {aka 'long long int'} and 'long unsigned int' [-Wsign-compare]
   67 |         } while (lptr == skip);
      |                  ~~~~~^~~~~~~
input/code.cpp:71:23: warning: comparison of integer expressions of different signedness: 'll' {aka 'long long int'} and 'long unsigned int' [-Wsign-compare]
   71 |         }...

// clang-format off
#include <bits/stdc++.h>
#include <optional>
using namespace std;
#ifdef DO_DBG
template<typename A, typename B> ostream& operator<<(ostream &os, const pair<A, B> &p) { return os << '(' << p.first << ", " << p.second << ')'; }
template<typename T_container, typename T = typename enable_if<!is_same<T_container, string>::value, typename T_container::value_type>::type> ostream& operator<<(ostream &os, const T_container &v) { os << '{'; string sep; for (const T &x : v) os << sep << x, sep = ", "; return os << '}'; }
void dbg_out() { cerr << endl; }
template<typename Head, typename... Tail> void dbg_out(Head H, Tail... T) { cerr << ' ' << H; dbg_out(T...); }
#define dbg(...) cerr << "(" << #__VA_ARGS__ << "):", dbg_out(__VA_ARGS__)
#define dout cerr
#define debug true
#else
#define dbg(...)
#define dout if (0) cerr
#define debug false
#endif
typedef long long ll; typedef unsigned long long ull; typedef long double ld;
template <typename T> using v = vector<T>; template <typename T> using us = unordered_set<T>; template <typename K, typename V> using p = pair<K,V>;
template <typename K, typename V> using um = unordered_map<K, V>; template <typename K, typename V> using p = pair<K, V>;
template <typename T> using pq = priority_queue<T>; template <typename T> using nl = numeric_limits<T>; template <typename T> using il = initializer_list<T>;
constexpr int MOD = 1e9 + 7; constexpr int INF = 1e9; constexpr ld EPS = 1e-9;
#define fr(i,a,b) for (size_t i = a; i < b; i++)
#define fastio ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL); cin.exceptions(cin.failbit);
#define frr(i,a,b) for (size_t i = b-1; i >= a; i--)
#define frs(i,a,b) for (ll i = a; i < b; i++)
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define sq(x) ((x) * (x))
template <typename... Args> void read(Args&... args) { ((cin >> args), ...); }
#define d(...) int __VA_ARGS__; read(__VA_ARGS__);
#define dll(...) ll __VA_ARGS__; read(__VA_ARGS__);
#define dv(x, n) vector<int> x(n); for (int i = 0; i < n; i++) cin >> x[i];
#define dvll(x, n) vector<long long> x(n); for (int i = 0; i < n; i++) cin >> x[i];
#define dvd(x, n) vector<double> x(n); for (int i = 0; i < n; i++) cin >> x[i];
ll ipow(ll a,int b){ll r=1;for(;b;b>>=1,a*=a)if(b&1)r*=a;return r;}
template<class F>struct y_combinator{F f;template<class...Args>decltype(auto)operator()(Args&&...args)const{return f(*this,forward<Args>(args)...);}};
constexpr auto get_nums=[]<typename T>(T&&s){istringstream iss(forward<T>(s));return vector<int>{istream_iterator<int>{iss},{}};};
// clang-format on

int main() {
  fastio;
  dll(n, x);
  if (n < 2) {
    cout << "IMPOSSIBLE\n";
    return 0;
  }
  v<p<ll, ll>> ap(n);
  fr(i, 0, n) {
    ll a1;
    cin >> a1;
    ap[i] = {a1, i + 1};
  }
  sort(all(ap));
  v<ll> a(n);
  transform(all(ap), a.begin(), [](auto p) { return p.first; });

  auto sum2 = [&](auto x, auto skip) {
    ll lptr = 0, rptr = a.size() - 1;
    while (lptr < rptr) {
      ll cursum = a[lptr] + a[rptr];
      if (cursum == x)
        break;
      else if (cursum < x) {
        do {
          lptr++;
        } while (lptr == skip);
      } else {
        do {
          rptr--;
        } while (rptr == skip);
      }
    }
    if (a[lptr] + a[rptr] == x && lptr < rptr && lptr != skip && rptr != skip) {
      dbg(x, a[lptr], a[rptr]);
      return optional{p{lptr, rptr}};
    } else {
      return optional<p<ll, ll>>{};
    }
  };

  fr(i, 0, a.size()) {
    ll need = x - a[i];
    dbg(need);
    auto s2 = sum2(need, i);
    if (s2.has_value()) {
      cout << ap[i].second << " " << ap[s2->first].second << " "
           << ap[s2->second].second << "\n";
      return 0;
    }
  }
  cout << "IMPOSSIBLE\n";
}

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