Problem p02691 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cstdio>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <math.h>
#include <queue>
#include <set>
#include <stdlib.h>
#include <string>
#include <utility>
#include <vector>
using namespace std;
typedef long long int ll;
typedef long double ld;
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
const long long INF = 1LL << 60;
typedef pair<ll, ll> pairs;
vector<pairs> p;
bool pairCompare(const pair<double, ll> &firstElof,
                 const pair<double, ll> &secondElof) {
  return firstElof.first < secondElof.first;
}
bool pairCompareSecond(const pair<double, ll> &firstElof,
                       const pair<double, ll> &secondElof) {
  return firstElof.second < secondElof.second;
}
#define MAX_N 100100
#define MOD 1000000007
bool x[MAX_N];
ll num[MAX_N];
ll fibl[MAX_N] = {0};
// 四方向への移動ベクトル
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

ll fib(ll a) {
  if (fibl[a] != 0)
    return fibl[a];
  if (a == 0) {
    return 0;
  } else if (a == 1) {
    return 1;
  }
  return fibl[a] = fib(a - 1) + fib(a - 2);
}

ll eratosthenes(ll n) {
  int p = 0;
  for (ll i = 0; i <= n; ++i)
    x[i] = true;
  x[0] = x[1] = false;
  for (int i = 2; i <= n; ++i) {
    if (x[i]) {
      p++;
      for (int j = 2 * i; j <= n; j += i)
        x[j] = false;
    }

    num[i] = p;
  }
  return p;
}

ll gcd(ll a, ll b) {
  if (a % b == 0)
    return (b);
  else
    return (gcd(b, a % b));
}

ll keta(ll N) {
  int tmp{};
  while (N > 0) {
    tmp += (N % 10);
    N /= 10;
  }
  N = tmp;
  return N;
}

void base_num(ll data, ll base) {
  if (data == 0)
    return;
  ll next = abs(data) % abs(base);
  if (data < 0)
    next = (abs(base) - next) % abs(base);
  base_num((data - next) / base, base);
  cout << next;
}

int main() {
  ll n, ans = 0;
  cin >> n;
  ll a[n], b[200100];
  fill(b, b + 200100, 0);
  for (ll i = 0; i < n; i++) {
    cin >> a[i];
    if (a[i] <= n) {
      b[a[i] + i + 1] = b[a[i] + i + 1] + 1;
    }
  }
  for (ll i = 0; i < n; i++) {
    if (i + 1 - a[i] >= 0) {
      ans += b[i + 1 - a[i]];
    }
  }
  cout << ans << endl;
  return 0;
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <cstdio>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <math.h>
#include <queue>
#include <set>
#include <stdlib.h>
#include <string>
#include <utility>
#include <vector>
using namespace std;
typedef long long int ll;
typedef long double ld;
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
const long long INF = 1LL << 60;
typedef pair<ll, ll> pairs;
vector<pairs> p;
bool pairCompare(const pair<double, ll> &firstElof,
                 const pair<double, ll> &secondElof) {
  return firstElof.first < secondElof.first;
}
bool pairCompareSecond(const pair<double, ll> &firstElof,
                       const pair<double, ll> &secondElof) {
  return firstElof.second < secondElof.second;
}
#define MAX_N 100100
#define MOD 1000000007
bool x[MAX_N];
ll num[MAX_N];
ll fibl[MAX_N] = {0};
// 四方向への移動ベクトル
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

ll fib(ll a) {
  if (fibl[a] != 0)
    return fibl[a];
  if (a == 0) {
    return 0;
  } else if (a == 1) {
    return 1;
  }
  return fibl[a] = fib(a - 1) + fib(a - 2);
}

ll eratosthenes(ll n) {
  int p = 0;
  for (ll i = 0; i <= n; ++i)
    x[i] = true;
  x[0] = x[1] = false;
  for (int i = 2; i <= n; ++i) {
    if (x[i]) {
      p++;
      for (int j = 2 * i; j <= n; j += i)
        x[j] = false;
    }

    num[i] = p;
  }
  return p;
}

ll gcd(ll a, ll b) {
  if (a % b == 0)
    return (b);
  else
    return (gcd(b, a % b));
}

ll keta(ll N) {
  int tmp{};
  while (N > 0) {
    tmp += (N % 10);
    N /= 10;
  }
  N = tmp;
  return N;
}

void base_num(ll data, ll base) {
  if (data == 0)
    return;
  ll next = abs(data) % abs(base);
  if (data < 0)
    next = (abs(base) - next) % abs(base);
  base_num((data - next) / base, base);
  cout << next;
}

int main() {
  ll n, ans = 0;
  cin >> n;
  ll a[n], b[200100];
  fill(b, b + 200100, 0);
  for (ll i = 0; i < n; i++) {
    cin >> a[i];
    if (a[i] + i + 1 < n) {
      b[a[i] + i + 1] = b[a[i] + i + 1] + 1;
    }
  }
  for (ll i = 0; i < n; i++) {
    if (i + 1 - a[i] >= 0) {
      ans += b[i + 1 - a[i]];
    }
  }
  cout << ans << endl;
  return 0;
}