Problem p02572 (C++) — buggy submission, verdict: Time Limit Exceeded

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

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<string, string> pss;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<pii> vii;
typedef vector<ll> vl;
typedef vector<vl> vvl;

double EPS = 1e-9;
int INF = 1000000005;
long long INFF = 1000000000000000005ll;
double PI = acos(-1);
int dirx[8] = {-1, 0, 0, 1, -1, -1, 1, 1};
int diry[8] = {0, 1, -1, 0, -1, 1, -1, 1};
const ll MOD = 1000000007;

#define DEBUG fprintf(stderr, "====TESTING====\\n")
#define VALUE(x) cerr << "The value of " << #x << " is " << x << endl
#define OUT(x) cout << x << endl
#define debug(...) fprintf(stderr, __VA_ARGS__)
#define READ(x)                                                                \\
  for (auto &(z) : x)                                                          \\
    cin >> z;
#define FOR(a, b, c) for (int(a) = (b); (a) < (c); ++(a))
#define FORN(a, b, c) for (int(a) = (b); (a) <= (c); ++(a))
#define FORD(a, b, c) for (int(a) = (b); (a) >= (c); --(a))
#define FORSQ(a, b, c) for (int(a) = (b); (a) * (a) <= (c); ++(a))
#define FORC(a, b, c) for (char(a) = (b); (a) <= (c); ++(a))
#define EACH(a, b) for (auto &(a) : (b))
#define REP(i, n) FOR(i, 0, n)
#define REPN(i, n) FORN(i, 1, n)
#define MAX(a, b) a = max(a, b)
#define MIN(a, b) a = min(a, b)
#define SQR(x) ((ll)(x) * (x))
#define RESET(a, b) memset(a, b, sizeof(a))
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define ALL(v) v.begin(), v.end()
#define ALLA(arr, sz) arr, arr + sz
#define SIZE(v) (int)v.size()
#define SORT(v) sort(ALL(v))
#define REVERSE(v) reverse(ALL(v))
#define SORTA(arr, sz) sort(ALLA(arr, sz))
#define REVERSEA(arr, sz) reverse(ALLA(arr, sz))
#define PERMUTE next_permutation
#define TC(t) while (t--)
#define FAST_INP                                                               \\
  ios_base::sync_with_stdio(false);                                            \\
  cin.tie(NULL)

void solve() {}

int main() {
  FAST_INP;
  //    #ifndef ONLINE_JUDGE
  //    freopen("input.txt","r", stdin);
  //    freopen("output.txt","w", stdout);
  //    #endif
  //
  //    int tc; cin >> tc;
  //    TC(tc) solve();
  int n;
  cin >> n;
  vl a(n);
  READ(a);
  sort(a.rbegin(), a.rend());
  ll sum = a[0] + a[1], ans = (a[0] * a[1]) % MOD;
  //    ll sum2=(sum*sum)%MOD, k=0;
  //    REP(i,n) {
  //    	a[i]%=MOD;
  //    	int aa=(a[i]*a[i])%MOD;
  //    	k=(k+aa)%MOD;
  //	}
  //	cout << sum2 << " " << k << endl;
  FOR(i, 2, n) {
    ans = (ans + (sum * a[i])) % MOD;
    sum = (sum + a[i]) % MOD;
  }
  OUT((ans % MOD));
  return 0;
}

Error: TLE

Fixed submission:

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

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<string, string> pss;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<pii> vii;
typedef vector<ll> vl;
typedef vector<vl> vvl;

double EPS = 1e-9;
int INF = 1000000005;
long long INFF = 1000000000000000005ll;
double PI = acos(-1);
int dirx[8] = {-1, 0, 0, 1, -1, -1, 1, 1};
int diry[8] = {0, 1, -1, 0, -1, 1, -1, 1};
const ll MOD = 1000000007;

#define DEBUG fprintf(stderr, "====TESTING====\\n")
#define VALUE(x) cerr << "The value of " << #x << " is " << x << endl
#define OUT(x) cout << x << endl
#define debug(...) fprintf(stderr, __VA_ARGS__)
#define READ(x)                                                                \\
  for (auto &(z) : x)                                                          \\
    cin >> z;
#define FOR(a, b, c) for (int(a) = (b); (a) < (c); ++(a))
#define FORN(a, b, c) for (int(a) = (b); (a) <= (c); ++(a))
#define FORD(a, b, c) for (int(a) = (b); (a) >= (c); --(a))
#define FORSQ(a, b, c) for (int(a) = (b); (a) * (a) <= (c); ++(a))
#define FORC(a, b, c) for (char(a) = (b); (a) <= (c); ++(a))
#define EACH(a, b) for (auto &(a) : (b))
#define REP(i, n) FOR(i, 0, n)
#define REPN(i, n) FORN(i, 1, n)
#define MAX(a, b) a = max(a, b)
#define MIN(a, b) a = min(a, b)
#define SQR(x) ((ll)(x) * (x))
#define RESET(a, b) memset(a, b, sizeof(a))
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define ALL(v) v.begin(), v.end()
#define ALLA(arr, sz) arr, arr + sz
#define SIZE(v) (int)v.size()
#define SORT(v) sort(ALL(v))
#define REVERSE(v) reverse(ALL(v))
#define SORTA(arr, sz) sort(ALLA(arr, sz))
#define REVERSEA(arr, sz) reverse(ALLA(arr, sz))
#define PERMUTE next_permutation
#define TC(t) while (t--)
#define FAST_INP                                                               \\
  ios_base::sync_with_stdio(false);                                            \\
  cin.tie(NULL)

void solve() {}

int main() {
  FAST_INP;
  //    #ifndef ONLINE_JUDGE
  //    freopen("input.txt","r", stdin);
  //    freopen("output.txt","w", stdout);
  //    #endif
  //
  //    int tc; cin >> tc;
  //    TC(tc) solve();
  int n;
  cin >> n;
  vl a(n);
  READ(a);
  ll ans = 0;
  REP(i, n) FOR(j, i + 1, n) {
    ll x = (a[i] * a[j]) % MOD;
    //		cout << "x:" << x << endl;
    ans = (ans + x) % MOD;
    //		cout << "ans:" << ans << endl;
  }
  OUT(ans);
  return 0;
}