Problem p02691 (C++)

Buggy submission (status: Time Limit Exceeded):
// Contest: AtCoder Beginner Contest 166 (https://atcoder.jp/contests/abc166)
// Problem: E: This Message Will Self-Destruct in 5s
// (https://atcoder.jp/contests/abc166/tasks/abc166_e)

// region {{{ boilerplate
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <chrono>
#include <cmath>
#include <cstdint>
#include <deque>
#include <exception>
#include <forward_list>
#include <functional>
#include <initializer_list>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <memory>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#if __cplusplus >= 201703L
#include <optional>
#include <string_view>
#include <variant>
#endif

using namespace std;

#if __cplusplus >= 201703L
#define all(a) begin(a), end(a)
#define sz(x) ((int)size(x))
#else
#define all(a) (a).begin(), (a).end()
#define sz(x) ((int)(x).size())
#endif

#define rep(a, b) for (int a = 0; a < (b); ++a)
#define reps(a, b, c) for (int a = (b); a < (c); ++a)
#define trav(a, b) for (auto &a : b)

using ll = long long;
using ld = long double;
using u64 = uint64_t;
using u32 = uint32_t;

using pi = pair<int, int>;
using pl = pair<ll, ll>;
using vi = vector<int>;
using vl = vector<ll>;
using vvi = vector<vi>;
using vpi = vector<pi>;

#if __cplusplus >= 201703L
#define MAYBE_UNUSED [[maybe_unused]]
#else
#define MAYBE_UNUSED
#endif

MAYBE_UNUSED static constexpr int INF = (int)1e9 + 5;
MAYBE_UNUSED static constexpr ll INFL = (ll)1e18 + 5;
MAYBE_UNUSED static mt19937
    rng((u32)chrono::duration_cast<chrono::nanoseconds>(
            chrono::high_resolution_clock::now().time_since_epoch())
            .count());
// endregion }}}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  int n;
  cin >> n;
  vi a(n);
  trav(ai, a) cin >> ai;

  ll cnt = 0;
  multiset<int> s;
  rep(i, n) cnt += s.count(i - a[i]), s.emplace(i + a[i]);
  cout << cnt << '\n';

  return 0;
}

Fixed submission:
// Contest: AtCoder Beginner Contest 166 (https://atcoder.jp/contests/abc166)
// Problem: E: This Message Will Self-Destruct in 5s
// (https://atcoder.jp/contests/abc166/tasks/abc166_e)

// region {{{ boilerplate
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <chrono>
#include <cmath>
#include <cstdint>
#include <deque>
#include <exception>
#include <forward_list>
#include <functional>
#include <initializer_list>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <memory>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#if __cplusplus >= 201703L
#include <optional>
#include <string_view>
#include <variant>
#endif

using namespace std;

#if __cplusplus >= 201703L
#define all(a) begin(a), end(a)
#define sz(x) ((int)size(x))
#else
#define all(a) (a).begin(), (a).end()
#define sz(x) ((int)(x).size())
#endif

#define rep(a, b) for (int a = 0; a < (b); ++a)
#define reps(a, b, c) for (int a = (b); a < (c); ++a)
#define trav(a, b) for (auto &a : b)

using ll = long long;
using ld = long double;
using u64 = uint64_t;
using u32 = uint32_t;

using pi = pair<int, int>;
using pl = pair<ll, ll>;
using vi = vector<int>;
using vl = vector<ll>;
using vvi = vector<vi>;
using vpi = vector<pi>;

#if __cplusplus >= 201703L
#define MAYBE_UNUSED [[maybe_unused]]
#else
#define MAYBE_UNUSED
#endif

MAYBE_UNUSED static constexpr int INF = (int)1e9 + 5;
MAYBE_UNUSED static constexpr ll INFL = (ll)1e18 + 5;
MAYBE_UNUSED static mt19937
    rng((u32)chrono::duration_cast<chrono::nanoseconds>(
            chrono::high_resolution_clock::now().time_since_epoch())
            .count());
// endregion }}}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  int n;
  cin >> n;
  vi a(n);
  trav(ai, a) cin >> ai;

  ll cnt = 0;
  map<int, int> s;
  rep(i, n) cnt += s[i - a[i]], s[i + a[i]]++;
  cout << cnt << '\n';

  return 0;
}