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

#include <bits/stdc++.h>
using namespace std;

using lnt = long long;
using Real = long double;

#define ALL(obj) (obj).begin(), (obj).end()
#define RALL(obj) (obj).rbegin(), (obj).rend()
#define REP(i, n) for (lnt i = 0; i < (n); ++i)
#define RANGE(i, a, b) for (lnt i = (a); i < (b); ++i)
#define RREP(i, n) for (lnt i = (n)-1; i >= 0; --i)

#define ln '\\n'
#define pb push_back
#define eb emplace_back
#define pque priority_queue
#define umap unordered_map
#define uset unordered_set
#define dcout cout << fixed << setprecision(20)
#define summon_tourist                                                         \\
  cin.tie(0);                                                                  \\
  ios::sync_with_stdio(false)

constexpr int dx[] = {1, 0, -1, 0, 1, 1, -1, -1},
              dy[] = {0, -1, 0, 1, 1, -1, 1, -1};
constexpr Real eps = 1e-9;
const Real pi = acosl(-1);
constexpr lnt BIG = INT_MAX / 10, BBIG = LLONG_MAX / 10;

template <class T> inline T GCD(T a, T b) {
  T c;
  while (b != 0) {
    c = a % b;
    a = b;
    b = c;
  }
  return a;
}
template <class T> inline T LCM(T a, T b) {
  T c = GCD(a, b);
  a /= c;
  return a * b;
}
template <class T> inline T nCr(T a, T b) {
  T i, r = 1;
  for (i = 1; i <= b; i++) {
    r *= (a + 1 - i);
    r /= i;
  }
  return r;
}
template <class T> inline T nHr(T a, T b) { return nCr(a + b - 1, b); }
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;
}

using pint = pair<int, int>;
using plnt = pair<lnt, lnt>;
using vint = vector<int>;
using vlnt = vector<lnt>;
constexpr lnt MOD = 1e9 + 7;

int main() {
  summon_tourist;

  lnt N, A[200000];
  cin >> N;
  map<lnt, lnt> B;
  REP(i, N) {
    cin >> A[i];
    B[i + 1 - A[i]]++;
  }
  lnt ans = 0;
  REP(i, N) {
    ans += B[i + 1 + A[i]];
    if (i + 1 - A[i] == i + 1 + A[i])
      ans--;
  }
  cout << ans << ln;
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>
using namespace std;

using lnt = long long;
using Real = long double;

#define ALL(obj) (obj).begin(), (obj).end()
#define RALL(obj) (obj).rbegin(), (obj).rend()
#define REP(i, n) for (lnt i = 0; i < (n); ++i)
#define RANGE(i, a, b) for (lnt i = (a); i < (b); ++i)
#define RREP(i, n) for (lnt i = (n)-1; i >= 0; --i)

#define ln '\\n'
#define pb push_back
#define eb emplace_back
#define pque priority_queue
#define umap unordered_map
#define uset unordered_set
#define dcout cout << fixed << setprecision(20)
#define summon_tourist                                                         \\
  cin.tie(0);                                                                  \\
  ios::sync_with_stdio(false)

constexpr int dx[] = {1, 0, -1, 0, 1, 1, -1, -1},
              dy[] = {0, -1, 0, 1, 1, -1, 1, -1};
constexpr Real eps = 1e-9;
const Real pi = acosl(-1);
constexpr lnt BIG = INT_MAX / 10, BBIG = LLONG_MAX / 10;

template <class T> inline T GCD(T a, T b) {
  T c;
  while (b != 0) {
    c = a % b;
    a = b;
    b = c;
  }
  return a;
}
template <class T> inline T LCM(T a, T b) {
  T c = GCD(a, b);
  a /= c;
  return a * b;
}
template <class T> inline T nCr(T a, T b) {
  T i, r = 1;
  for (i = 1; i <= b; i++) {
    r *= (a + 1 - i);
    r /= i;
  }
  return r;
}
template <class T> inline T nHr(T a, T b) { return nCr(a + b - 1, b); }
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;
}

using pint = pair<int, int>;
using plnt = pair<lnt, lnt>;
using vint = vector<int>;
using vlnt = vector<lnt>;
constexpr lnt MOD = 1e9 + 7;

int main() {
  summon_tourist;

  lnt N, A[200000];
  cin >> N;
  multiset<lnt> B;
  REP(i, N) {
    cin >> A[i];
    B.insert(i + 1 - A[i]);
  }
  int ans = 0;
  REP(i, N) {
    ans += B.count(i + 1 + A[i]);
    if (i + 1 - A[i] == i + 1 + A[i])
      ans--;
  }
  cout << ans << ln;
}