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

///   |----------------------------|
///   | BISMILLAHIR RAHMANIR RAHIM |
///   |----------------------------|

///       _______  __   ________   ___   .___________.
///      /       ||  | |   ____|  /   \\  |           |
///     |   (----'|  | |  |__    /  ^  \\ `---|  |----`
///      \\   \\    |  | |   __|  /  /_\\  \\    |  |
///  |----)   |   |  | |  |    /  _____  \\   |  |
///  |_______/    |__| |__|   /__/     \\__\\  |__|
///

// #define BitCnt(a) __builtin_popcountll(a)
// template <typename T> T in(){char ch;T n = 0;bool ng = false;while (1){ch =
// getchar();if (ch == '-'){ng = true;ch = getchar();break;}if (ch>='0' &&
// ch<='9')     break;}while (1){n = n*10 + (ch - '0');ch = getchar();if (ch<'0'
// || ch>'9')   break;}return (ng?-n:n);} template<typename T>inline T POW(T B,T
// P){ if(P==0) return 1; if(P&1) return B*POW(B,P-1);  else return
// SQR(POW(B,P/2));} template<typename T>inline T Gcd(T a,T b){if(a<0)return
// Gcd(-a,b);if(b<0)return Gcd(a,-b);return (b==0)?a:Gcd(b,a%b);}
// template<typename T>inline T Lcm(T a,T b) {if(a<0)return
// Lcm(-a,b);if(b<0)return Lcm(a,-b);return a*(b/Gcd(a,b));} long long
// Bigmod(long long base, long long power, long long MOD){long long
// ret=1;while(power){if(power &
// 1)ret=(ret*base)%MOD;base=(base*base)%MOD;power>>=1;}return ret;} bool
// isVowel(char ch){ ch=toupper(ch);
// if(ch=='A'||ch=='U'||ch=='I'||ch=='O'||ch=='E') return true; return false;}
// long long ModInverse(long long number, long long MOD){return Bigmod(number,
// MOD-2, MOD);} template <typename T> LL isLeft(T a,T b,T c)
// {return(a.x-b.x)*(b.y-c.y)-(b.x-c.x)*(a.y-b.y);} bool isConst(char ch){if
// (isalpha(ch) && !isVowel(ch)) return true; return false;} int toInt(string s)
// { int sm; stringstream ss(s); ss>>sm; return sm; } #define good st<=l&&ed>=r
// #define seg    int mid=(l+r)/2,lft=2*id,rgt=lft+1
// #define MX  500007
///**********************************************************//
#include <bits/stdc++.h>
using namespace std;

#define ff first
#define ss second
#define pb push_back
#define mp make_pair

#define SQ(n) (n * n)
#define ll long long
#define llu unsigned long long

#define pi acos(-1.0)

#define all(a) a.begin(), a.end()
#define _max(a, b, c) Max(a, Max(b, c))
#define _min(a, b, c) Min(a, Min(b, c))

#define FOR(i, a, b) for (int i = a; i < b; i++)

#define ms_inf(a) memset(a, 0x3f, sizeof(a))
#define ms_ninf(a) memset(a, 0xcf, sizeof(a))
#define MEM(a, val) memset(a, val, sizeof(a))

#define sf(a) scanf("%d", &a)
#define sf2(a, b) scanf("%d%d", &a, &b)
#define sf3(a, b, c) scanf("%d%d%d", &a, &b, &c)
#define sf4(a, b, c, d) scanf("%d%d%d%d", &a, &b, &c, &d)

#define sfl(a) scanf("%lld", &a)
#define sfl2(a, b) scanf("%lld%lld", &a, &b)
#define sfl3(a, b, c) scanf("%lld%lld%lld", &a, &b, &c)
#define sfl4(a, b, c, d) scanf("%lld%lld%lld%lld", &a, &b, &c, &d)

#define pnl() printf("\\n")
#define pfs() printf(" ")

#define pfc(a) printf("Case %d: ", a)
#define pf(a) printf("%d", a)
#define pf2(a, b) printf("%d %d", a, b)
#define pf3(a, b, c) printf("%d %d %d", a, b, c)
#define pf4(a, b, c, d) printf("%d %d %d %d", a, b, c, d)

#define pflc(a) printf("Case %lld: ", a)
#define pfl(a) printf("%lld", a)
#define pfl2(a, b) printf("%lld %lld", a, b)
#define pfl3(a, b, c) printf("%lld %lld %lld", a, b, c)
#define pfl4(a, b, c, d) printf("%lld %lld %lld %lld", a, b, c, d)

#define tover(QLOW, QHIGH, LOW, HIGH)                                          \\
  QLOW <= LOW &&QHIGH >= HIGH /// QLOW,QHIGH = Jei Query Ber korbo
#define nover(QLOW, QHIGH, LOW, HIGH) QLOW > HIGH || QHIGH < LOW

#define bitCheck(a, k) ((bool)(a & (1 << (k))))
#define bitOff(a, k) (a & (~(1 << (k))))
#define bitOn(a, k) (a | (1 << (k)))
#define bitFlip(a, k) (a ^ (1 << (k)))

#define POPCOUNT __builtin_popcount
#define POPCOUNTLL __builtin_popcount
#define RIGHTMOST __builtin_ctzll
#define LEFTMOST(x) (63 - __builtin_clzll((x)))

#define pii pair<int, int>
#define pll pair<ll, ll>
#define vi vector<int>
#define FI freopen("in.txt", "r", stdin)
#define FO freopen("out.txt", "w", stdout)
#define MOD 1000000007
#define inf INT_MAX / 3

int fx[] = {0, 1, -1, 0, 0, 1, 1, -1, -1};
int fy[] = {0, 0, 0, 1, -1, 1, -1, -1, 1};
/// same, down, up, right, left, down-right, down-left, up-left, up-right

#define fastio()                                                               \\
  ios_base::sync_with_stdio(0);                                                \\
  cin.tie(0);                                                                  \\
  cout.tie(0)
int main() {
  ll n;
  sfl(n);
  ll arr[n + 1];
  ll sum = 0;
  for (int i = 0; i < n; i++) {
    sfl(arr[i]);
    sum += arr[i];
  }
  ll res = 0;
  for (int i = 0; i < n; i++) {
    sum -= arr[i];
    res += ((sum % MOD) * (arr[i] % MOD)) % MOD;
    res %= MOD;
  }
  pfl(res);
  pnl();
}


Error: TLE

Fixed submission:

#include <bits/stdc++.h>
#define ll long long
#define pb push_back
#define pf push_front
using namespace std;

int N = 1e5 + 7;

ll N1 = 1e9 + 7;

int main() {

  ll TJK = 1;

  // cin >> TJK ;

  while (TJK--) {

    ll n;

    cin >> n;

    ll ans = 0;

    ll a[n];

    for (int i = 0; i < n; i++) {

      cin >> a[i];
    }

    for (int i = 0; i < n - 1; i++) {

      for (int j = i + 1; j < n; j++) {

        ans %= N1;

        ans += (a[i] * a[j]);

        ans %= N1;
      }
    }

    cout << ans << endl;
  }
}