Problem p00452 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
#define FOR(i, s, n) for (int i = s; i < (int)n; ++i)
#define per(i, n) for (int i = n; i >= 0; i--)
#define ROF(i, s, n) for (int i = s; i >= (int)n; i--)
#define FORIT(i, A) for (auto i : A)
#define PRINT(x) cout << (x) << "\n"
#define SCAN(x) cin >> x
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define MP make_pair
#define PB push_back
#define EACH(i, n)                                                             \
  for (__typeof((n).begin()) i = (n).begin(); i != (n).end(); ++i)
#define SZ(a) int((a).size())
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())
#define CLR(a) memset((a), 0, sizeof(a))
#define NCLR(a) memset((a), -1, sizeof(a))
#define sq(n) (n) * (n)
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef complex<long double> P;
typedef long long LL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef priority_queue<int> maxpq;
typedef priority_queue<int, vector<int>, greater<int>> minpq;
typedef complex<double> Point;
static const double EPS = 1e-10;
static const double PI = acos(-1.0);
static const int mod = 1000000007;
static const int INF = 1 << 29;
static const LL LL_INF = 1ll << 60;
static const int dx[] = {-1, 0, 1, 0, 1, -1, 1, -1};
static const int dy[] = {0, -1, 0, 1, 1, 1, -1, -1};

#ifdef WIN32
#define dump(x) cerr << #x << " = " << (x) << "\n"
#define debug(x)                                                               \
  cerr << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << "\n"
#else
#define dump(x)
#define debug(x)
#endif

template <class T> ostream &operator<<(ostream &os, vector<T> &v) {
  for (int i = 0; i < v.size(); i++) {
    if (i)
      os << " ";
    os << v[i];
  }
  return os;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  while (cin >> n >> m && n && m) {
    VI p(n);
    for (int i = 0; i < n; i++) {
      cin >> p[i];
    }
    p.push_back(0);
    SORT(p);
    set<int> two_selected;
    for (int i = 0; i < n + 1; i++) {
      for (int j = 0; j < n + 1; j++) {
        two_selected.insert(p[i] + p[j]);
      }
    }
    int ans = 0;
    for (set<int>::iterator it = two_selected.begin(); it != two_selected.end();
         ++it) {
      set<int>::iterator it2 = two_selected.upper_bound(m - *it);
      if (it2 != two_selected.begin()) {
        --it2;
        ans = max(ans, *it + *it2);
      }
    }
    PRINT(ans);
  }
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define FOR(i, s, n) for (int i = s; i < (int)n; ++i)
#define per(i, n) for (int i = n; i >= 0; i--)
#define ROF(i, s, n) for (int i = s; i >= (int)n; i--)
#define FORIT(i, A) for (auto i : A)
#define PRINT(x) cout << (x) << "\n"
#define SCAN(x) cin >> x
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define MP make_pair
#define PB push_back
#define EACH(i, n)                                                             \
  for (__typeof((n).begin()) i = (n).begin(); i != (n).end(); ++i)
#define SZ(a) int((a).size())
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())
#define CLR(a) memset((a), 0, sizeof(a))
#define NCLR(a) memset((a), -1, sizeof(a))
#define sq(n) (n) * (n)
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef complex<long double> P;
typedef long long LL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef priority_queue<int> maxpq;
typedef priority_queue<int, vector<int>, greater<int>> minpq;
typedef complex<double> Point;
static const double EPS = 1e-10;
static const double PI = acos(-1.0);
static const int mod = 1000000007;
static const int INF = 1 << 29;
static const LL LL_INF = 1ll << 60;
static const int dx[] = {-1, 0, 1, 0, 1, -1, 1, -1};
static const int dy[] = {0, -1, 0, 1, 1, 1, -1, -1};

#ifdef WIN32
#define dump(x) cerr << #x << " = " << (x) << "\n"
#define debug(x)                                                               \
  cerr << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << "\n"
#else
#define dump(x)
#define debug(x)
#endif

template <class T> ostream &operator<<(ostream &os, vector<T> &v) {
  for (int i = 0; i < v.size(); i++) {
    if (i)
      os << " ";
    os << v[i];
  }
  return os;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  while (cin >> n >> m && n && m) {
    VI p(n);
    for (int i = 0; i < n; i++) {
      cin >> p[i];
    }
    p.push_back(0);
    SORT(p);
    set<int> two_selected;
    for (int i = 0; i < n + 1; i++) {
      for (int j = i; j < n + 1; j++) {
        two_selected.insert(p[i] + p[j]);
      }
    }
    int ans = 0;
    for (set<int>::iterator it = two_selected.begin(); it != two_selected.end();
         ++it) {
      set<int>::iterator it2 = two_selected.upper_bound(m - *it);
      if (it2 != two_selected.begin()) {
        --it2;
        ans = max(ans, *it + *it2);
      }
    }
    PRINT(ans);
  }
  return 0;
}