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

#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef long double ld;
#define REP(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define REPR(i, n) for (int i = n; i > -1; --i)
#define ALL(a) (a).begin(), (a).end()
#define FILL(a, n, x)                                                          \\
  ;                                                                            \\
  REP(i, (n)) { (a)[i] = (x); }
#define CINA(a, n)                                                             \\
  ;                                                                            \\
  REP(i, (n)) { cin >> (a)[i]; }
#define FILL2(a, n, m, x)                                                      \\
  ;                                                                            \\
  REP(i, (n)) {                                                                \\
    REP(j, (m)) { (a)[i][j] = (x); }                                           \\
  }
#define CINA2(a, n, m)                                                         \\
  ;                                                                            \\
  REP(i, (n)) {                                                                \\
    REP(j, (m)) { cin >> (a)[i][j]; }                                          \\
  }
#define Liny "Yes\\n"
#define Linn "No\\n"
#define LINY "YES\\n"
#define LINN "NO\\n"

#define umap unordered_map
// cout << setfill('0') << right << setw(4) << 12; // "0012"
int keta(ll x) {
  if (x < 10) {
    return 1;
  } else {
    return keta(x / 10) + 1;
  }
}
int keta_wa(ll x) {
  if (x < 10) {
    return x;
  } else {
    return keta_wa(x / 10) + x % 10;
  }
}
int ctoi(char c) { return ((c >= '0' && c <= '9') ? c - '0' : 0); }
int __stoi(string s) { return atoi(s.c_str()); }
ll sum(ll a[], ll N) { return accumulate(a, a + N, 0LL); }
ll gcd(ll a, ll b) {
  if (a < b)
    swap(a, b);
  return b ? gcd(b, a % b) : a;
}
ll lcm(ll a, ll b) {
  if (a < b) {
    swap(a, b);
  }
  return a / gcd(a, b) * b;
}
template <class T> void chmax(T &a, T b) {
  if (a < b) {
    a = b;
  }
}
template <class T> void chmin(T &a, T b) {
  if (a > b) {
    a = b;
  }
}

int main() {
  ll N;
  cin >> N;
  ll a[N + 1];
  umap<ll, ll> x, y;
  vector<ll> v;
  ll ans = 0;
  for (int i = 1; i <= N; ++i) {
    cin >> a[i];
    ll s = a[i] + i;
    ll t = i - a[i];
    ans += x[t];
    ++x[s];
  }

  cout << ans << "\\n";
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef long double ld;
#define REP(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define REPR(i, n) for (int i = n; i > -1; --i)
#define ALL(a) (a).begin(), (a).end()
#define FILL(a, n, x)                                                          \\
  ;                                                                            \\
  REP(i, (n)) { (a)[i] = (x); }
#define CINA(a, n)                                                             \\
  ;                                                                            \\
  REP(i, (n)) { cin >> (a)[i]; }
#define FILL2(a, n, m, x)                                                      \\
  ;                                                                            \\
  REP(i, (n)) {                                                                \\
    REP(j, (m)) { (a)[i][j] = (x); }                                           \\
  }
#define CINA2(a, n, m)                                                         \\
  ;                                                                            \\
  REP(i, (n)) {                                                                \\
    REP(j, (m)) { cin >> (a)[i][j]; }                                          \\
  }
#define Liny "Yes\\n"
#define Linn "No\\n"
#define LINY "YES\\n"
#define LINN "NO\\n"

#define umap unordered_map
// cout << setfill('0') << right << setw(4) << 12; // "0012"
int keta(ll x) {
  if (x < 10) {
    return 1;
  } else {
    return keta(x / 10) + 1;
  }
}
int keta_wa(ll x) {
  if (x < 10) {
    return x;
  } else {
    return keta_wa(x / 10) + x % 10;
  }
}
int ctoi(char c) { return ((c >= '0' && c <= '9') ? c - '0' : 0); }
int __stoi(string s) { return atoi(s.c_str()); }
ll sum(ll a[], ll N) { return accumulate(a, a + N, 0LL); }
ll gcd(ll a, ll b) {
  if (a < b)
    swap(a, b);
  return b ? gcd(b, a % b) : a;
}
ll lcm(ll a, ll b) {
  if (a < b) {
    swap(a, b);
  }
  return a / gcd(a, b) * b;
}
template <class T> void chmax(T &a, T b) {
  if (a < b) {
    a = b;
  }
}
template <class T> void chmin(T &a, T b) {
  if (a > b) {
    a = b;
  }
}

int main() {
  ll N;
  cin >> N;
  ll a[N + 1];
  for (int i = 1; i <= N; ++i) {
    cin >> a[i];
  }
  ll ans = 0;
  for (int i = 1; i < N; ++i) {
    for (int j = i + a[i] + 1; j <= N; ++j) {
      if (j - i == a[i] + a[j]) {
        ++ans;
      }
    }
  }
  cout << ans << "\\n";
}