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

/*input
32
3 1 4 1 5 9 2 6 5 3 5 8 9 7 9 3 2 3 8 4 6 2 6 4 3 3 8 3 2 7 9 5
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef tree<long long, null_type, less_equal<long long>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set;
#pragma GCC optimize("unroll-loops,no-stack-protector")
// order_of_key #of elements less than x
//  find_by_order kth element
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
#define f first
#define s second
#define pb push_back
#define REP(i, n) for (int i = 0; i < n; i++)
#define REP1(i, n) for (int i = 1; i <= n; i++)
#define FILL(n, x) memset(n, x, sizeof(n))
#define ALL(_a) _a.begin(), _a.end()
#define sz(x) (int)x.size()
#define SORT_UNIQUE(c)                                                         \\
  (sort(c.begin(), c.end()),                                                   \\
   c.resize(distance(c.begin(), unique(c.begin(), c.end()))))
#define MP make_pair
const ll INF64 = 4e10;
const int INF = 0x3f3f3f3f;
const ll MOD = 1e9 + 7;
const ld PI = acos(-1);
const ld eps = 1e-9;
#define lowb(x) x & (-x)
#define MNTO(x, y) x = min(x, (__typeof__(x))y)
#define MXTO(x, y) x = max(x, (__typeof__(x))y)
ll mult(ll a, ll b) { return ((a % MOD) * (b % MOD)) % MOD; }
ll mypow(ll a, ll b) {
  if (b <= 0)
    return 1;
  ll res = 1LL;
  while (b) {
    if (b & 1)
      res = mult(res, a);
    a = mult(a, a);
    b >>= 1;
  }
  return res;
}
const ll maxn = 1e3 + 5;
int32_t main() {
  ios::sync_with_stdio(false), cin.tie(0);
  int n;
  cin >> n;
  map<int, int> mp;
  ll ans = 0;
  REP1(i, n) {
    int x;
    cin >> x;
    ans += mp[i - x];
    mp[x + i]++;
  }
  cout << ans << '\\n';
}

Error: TLE

Fixed submission:

/*input
32
3 1 4 1 5 9 2 6 5 3 5 8 9 7 9 3 2 3 8 4 6 2 6 4 3 3 8 3 2 7 9 5
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef tree<long long, null_type, less_equal<long long>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set;
// #pragma GCC optimize("unroll-loops,no-stack-protector")
// order_of_key #of elements less than x
//  find_by_order kth element
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
#define f first
#define s second
#define pb push_back
#define REP(i, n) for (int i = 0; i < n; i++)
#define REP1(i, n) for (int i = 1; i <= n; i++)
#define FILL(n, x) memset(n, x, sizeof(n))
#define ALL(_a) _a.begin(), _a.end()
#define sz(x) (int)x.size()
#define SORT_UNIQUE(c)                                                         \\
  (sort(c.begin(), c.end()),                                                   \\
   c.resize(distance(c.begin(), unique(c.begin(), c.end()))))
#define MP make_pair
const ll INF64 = 4e10;
const int INF = 0x3f3f3f3f;
const ll MOD = 1e9 + 7;
const ld PI = acos(-1);
const ld eps = 1e-9;
#define lowb(x) x & (-x)
#define MNTO(x, y) x = min(x, (__typeof__(x))y)
#define MXTO(x, y) x = max(x, (__typeof__(x))y)
ll mult(ll a, ll b) { return ((a % MOD) * (b % MOD)) % MOD; }
ll mypow(ll a, ll b) {
  if (b <= 0)
    return 1;
  ll res = 1LL;
  while (b) {
    if (b & 1)
      res = mult(res, a);
    a = mult(a, a);
    b >>= 1;
  }
  return res;
}
const ll maxn = 1e3 + 5;
int32_t main() {
  ios::sync_with_stdio(false), cin.tie(0);
  int n;
  cin >> n;
  multiset<int> s;
  ll ans = 0;
  REP1(i, n) {
    int x;
    cin >> x;
    ans += s.count(i - x);
    s.insert(x + i);
  }
  cout << ans << '\\n';
}