Problem p02572 (C++)

Buggy submission (status: Runtime Error):
#include "bits/stdc++.h"
using namespace std;
#define int long long
#define IOS                                                                    \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(NULL);                                                               \
  cout.tie(NULL);
#define vi vector<int>
#define ff first
#define ss second
#define vp vector<pair<int, int>>
#define vpp vector<pair<int, pair<int, int>>>
#define seti set<int>
#define setbit(x) __builtin_popcountll(x)
#define sov(v) accumulate(all(v), 0)
#define fs(it, a) for (auto it = a.begin(); it != a.end(); it++)
#define pb push_back
#define pob pop_back
#define mp make_pair
#define pqmax priority_queue<int, vector<int>>
#define pqmin priority_queue<int, vector<int>, greater<int>>
#define dq deque<int>
#define umi unordered_map<int, int>
#define ums unordered_map<string, int>
#define ps(x, y) fixed << setprecision(y) << x
#define all(x) x.begin(), x.end()
#define f(x, y, z) for (x = y; x < z; x++)
#define si size()
#define ps(x, y) fixed << setprecision(y) << x
#define countdigit(x) floor(log10(x) + 1)
#define M 1000000007
#define PI 3.1415926535
// Use (k%M+m)%m always where k is any no
#define ee "\n"
#define re return

// Author Rahul Sannigrahi

vector<int> take(int n) {
  int i, j;
  vi v;
  f(i, 0, n) {
    cin >> j;
    v.pb(j);
  }
  return v;
}

void show(vector<int> v) {
  int i;
  for (i = 0; i < v.si; i++) {
    cout << v[i] << " ";
  }
  cout << ee;
}

int decode() {
  int i, j, k, l, n, m;
  cin >> n;
  vi v = take(n);
  vi pes(n);
  pes[v.si - 1] = v[v.si - 1];

  for (i = v.si - 2; i >= 0; i--) {
    pes[i] = pes[i + 1] + v[i];
  }
  // show(pes);
  l = 0;
  f(i, 0, v.si - 1) l = (l % M + ((v[i] % M) * (pes[i + 1] % M) + M)) % M;
  cout << l << ee;
  re 0;
}

int32_t main() {
  IOS
      // #ifndef ONLINE_JUDGE
      // freopen("input.txt","r",stdin);
      //  freopen("output.txt","w",stdout);
      // #endif
      int t = 1;
  cin >> t;
  while (t--)
    decode();
  return 0;
}


Fixed submission:
#include "bits/stdc++.h"
using namespace std;
#define int long long
#define IOS                                                                    \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(NULL);                                                               \
  cout.tie(NULL);
#define vi vector<int>
#define ff first
#define ss second
#define vp vector<pair<int, int>>
#define vpp vector<pair<int, pair<int, int>>>
#define seti set<int>
#define setbit(x) __builtin_popcountll(x)
#define sov(v) accumulate(all(v), 0)
#define fs(it, a) for (auto it = a.begin(); it != a.end(); it++)
#define pb push_back
#define pob pop_back
#define mp make_pair
#define pqmax priority_queue<int, vector<int>>
#define pqmin priority_queue<int, vector<int>, greater<int>>
#define dq deque<int>
#define umi unordered_map<int, int>
#define ums unordered_map<string, int>
#define ps(x, y) fixed << setprecision(y) << x
#define all(x) x.begin(), x.end()
#define f(x, y, z) for (x = y; x < z; x++)
#define si size()
#define ps(x, y) fixed << setprecision(y) << x
#define countdigit(x) floor(log10(x) + 1)
#define M 1000000007
#define PI 3.1415926535
// Use (k%M+m)%m always where k is any no
#define ee "\n"
#define re return

// Author Rahul Sannigrahi

vector<int> take(int n) {
  int i, j;
  vi v;
  f(i, 0, n) {
    cin >> j;
    v.pb(j);
  }
  return v;
}

void show(vector<int> v) {
  int i;
  for (i = 0; i < v.si; i++) {
    cout << v[i] << " ";
  }
  cout << ee;
}

int decode() {
  int i, j, k, l, n, m;
  cin >> n;
  vi v = take(n);
  vi pes(n);
  pes[v.si - 1] = v[v.si - 1];

  for (i = v.si - 2; i >= 0; i--) {
    pes[i] = pes[i + 1] + v[i];
  }
  // show(pes);
  l = 0;
  f(i, 0, v.si - 1) l = (l % M + ((v[i] % M) * (pes[i + 1] % M) + M)) % M;
  cout << l << ee;
  re 0;
}

int32_t main() {
  IOS
      // #ifndef ONLINE_JUDGE
      // freopen("input.txt","r",stdin);
      //  freopen("output.txt","w",stdout);
      // #endif
      int t = 1;
  // cin>>t;
  while (t--)
    decode();
  return 0;
}