Problem p02691 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using vl = vector<ll>;
template <class T> using vc = vector<T>;
template <class T> using vvc = vector<vector<T>>;

#define eb emplace_back
#define all(x) (x).begin(), (x).end()
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define repr(i, n) for (ll i = (n)-1; i >= 0; i--)
#define repe(i, l, r) for (ll i = (l); i < (r); i++)
#define reper(i, l, r) for (ll i = (r)-1; i >= (l); i--)
#define repa(i, n) for (auto &i : n)

template <class T> inline bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
void init() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
}

#ifdef DEBUG
template <class T, class N> void verr(const T &a, const N &n) {
  rep(i, n) cerr << a[i] << " ";
  cerr << "\\n" << flush;
}
ll dbgt = 1;
void err() { cerr << "passed " << dbgt++ << "\\n" << flush; }
template <class H, class... T> void err(H &&h, T &&...t) {
  cerr << h << (sizeof...(t) ? " " : "\\n") << flush;
  if (sizeof...(t) > 0)
    err(forward<T>(t)...);
}
#endif

const ll INF = 5e18;
const ld EPS = 1e-11;
const ld PI = acos(-1.0L);
const ll MOD = 1e9 + 7;
// const ll MOD = 998244353;
//--------------------------------------------------------------------------------//
ll A[200005], T[200005];
int main() {
  init();
  ll N;
  cin >> N;
  rep(i, N) cin >> A[i];
  if (N == 2) {
    cout << 0 << endl;
    return 0;
  }

  rep(i, N) T[i] = A[i] - i;
  map<ll, ll> mp;
  repe(i, 1, N) mp[T[i]]++;

  ll ans = 0;
  rep(i, N - 2) {
    mp[T[i + 1]]--;
    ans += mp[T[i] - 2 * A[i]];
  }

  cout << ans << endl;
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using vl = vector<ll>;
template <class T> using vc = vector<T>;
template <class T> using vvc = vector<vector<T>>;

#define eb emplace_back
#define all(x) (x).begin(), (x).end()
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define repr(i, n) for (ll i = (n)-1; i >= 0; i--)
#define repe(i, l, r) for (ll i = (l); i < (r); i++)
#define reper(i, l, r) for (ll i = (r)-1; i >= (l); i--)
#define repa(i, n) for (auto &i : n)

template <class T> inline bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
void init() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
}

#ifdef DEBUG
template <class T, class N> void verr(const T &a, const N &n) {
  rep(i, n) cerr << a[i] << " ";
  cerr << "\\n" << flush;
}
ll dbgt = 1;
void err() { cerr << "passed " << dbgt++ << "\\n" << flush; }
template <class H, class... T> void err(H &&h, T &&...t) {
  cerr << h << (sizeof...(t) ? " " : "\\n") << flush;
  if (sizeof...(t) > 0)
    err(forward<T>(t)...);
}
#endif

const ll INF = 5e18;
const ld EPS = 1e-11;
const ld PI = acos(-1.0L);
const ll MOD = 1e9 + 7;
// const ll MOD = 998244353;
//--------------------------------------------------------------------------------//
ll A[200005], T[200005];
int main() {
  init();
  ll N;
  cin >> N;
  rep(i, N) cin >> A[i];
  if (N == 2) {
    cout << 0 << endl;
    return 0;
  }

  rep(i, N) T[i] = A[i] - i;
  multiset<ll> st;
  repe(i, 1, N) st.emplace(T[i]);

  ll ans = 0;
  rep(i, N - 2) {
    st.erase(T[i + 1]);
    ans += st.count(T[i] - 2 * A[i]);
  }

  cout << ans << endl;
}