Problem p02691 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
typedef long long ll;
#define pb push_back
#define mp make_pair
#define all(a) (a).begin(), (a).end()
#define clr(a, h) memset(a, (h), sizeof(a))
#define mem(a, h) memset(a, (h), sizeof(a))
#define fi first
#define se second
#define por(a, b) (((a % MOD) * (b % MOD)) % MOD)
#define forg(i, b, e, c) for (ll i = (ll)b; i < (ll)e; i += c)
#define forr(i, b, e) for (ll i = b; i < e; i++)

using namespace std;
// using namespace __gnu_pbds;
typedef double lldb;
typedef pair<ll, ll> ii;
typedef pair<ii, ll> iii;
typedef vector<ll> vi;
typedef vector<vi> vvi;
typedef vector<ii> vii;
typedef vector<ll> vll;
// typedef
// tree<ll,null_type,less<ll>,rb_tree_tag,tree_order_statistics_node_update>
// ordered_set;
const ll INF = 1e9;
const double PI = acos(-1);
#define tam 120000
#define offset
const ll MOD = 1e9 + 7;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int suffix[tam], prefix[tam], v[tam];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  forr(i, 0, n) {
    cin >> v[i];
    if (i - (v[i] - 1) >= 0)
      suffix[i - (v[i] - 1)]++;
  }
  ll tot = 0;
  forr(i, 0, n) {

    int possuf = (i - (v[i] - 1));
    int pospre = (i + (v[i] - 1));
    if (possuf >= 0)
      suffix[possuf]--;
    if (i + v[i] + 1 < n)
      tot += suffix[i + v[i] + 1];

    if (i - v[i] - 1 >= 0)
      tot += prefix[i - v[i] - 1];
    if (pospre < n)
      prefix[pospre]++;
    //   cout<<tot<<' ';
  }
  cout << tot / 2 << endl;
}
// PLUS ULTRA!


Fixed submission:
#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
typedef long long ll;
#define pb push_back
#define mp make_pair
#define all(a) (a).begin(), (a).end()
#define clr(a, h) memset(a, (h), sizeof(a))
#define mem(a, h) memset(a, (h), sizeof(a))
#define fi first
#define se second
#define por(a, b) (((a % MOD) * (b % MOD)) % MOD)
#define forg(i, b, e, c) for (ll i = (ll)b; i < (ll)e; i += c)
#define forr(i, b, e) for (ll i = b; i < e; i++)

using namespace std;
// using namespace __gnu_pbds;
typedef double lldb;
typedef pair<ll, ll> ii;
typedef pair<ii, ll> iii;
typedef vector<ll> vi;
typedef vector<vi> vvi;
typedef vector<ii> vii;
typedef vector<ll> vll;
// typedef
// tree<ll,null_type,less<ll>,rb_tree_tag,tree_order_statistics_node_update>
// ordered_set;
const ll INF = 1e9;
const double PI = acos(-1);
#define tam 220000
#define offset
const ll MOD = 1e9 + 7;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int suffix[tam], prefix[tam], v[tam];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  forr(i, 0, n) {
    cin >> v[i];
    if (i - (v[i] - 1) >= 0)
      suffix[i - (v[i] - 1)]++;
  }
  ll tot = 0;
  forr(i, 0, n) {

    int possuf = (i - (v[i] - 1));
    int pospre = (i + (v[i] - 1));
    if (possuf >= 0)
      suffix[possuf]--;
    if (i + v[i] + 1 < n)
      tot += suffix[i + v[i] + 1];

    if (i - v[i] - 1 >= 0)
      tot += prefix[i - v[i] - 1];
    if (pospre < n)
      prefix[pospre]++;
    //   cout<<tot<<' ';
  }
  cout << tot / 2 << endl;
}
// PLUS ULTRA!