Problem p03201 (C++)

Buggy submission (status: Time Limit Exceeded):
/*
confirm 0LL and 1LL
confirm cornercases such as 0
confirm times of cin < 10^6
*/
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using P = pair<ll, ll>;
using Pld = pair<ld, ld>;
using Vec = vector<ll>;
using VecP = vector<P>;
using VecB = vector<bool>;
using VecC = vector<char>;
using VecD = vector<ld>;
using VecS = vector<string>;
using VecVec = vector<Vec>;
using Graph = vector<VecP>;
template <typename T> using Vec1 = vector<T>;
template <typename T> using Vec2 = vector<Vec1<T>>;
#define REP(i, m, n) for (ll(i) = (m); (i) < (n); ++(i))
#define REPN(i, m, n) for (ll(i) = (m); (i) <= (n); ++(i))
#define REPR(i, m, n) for (ll(i) = (m)-1; (i) >= (n); --(i))
#define REPNR(i, m, n) for (ll(i) = (m); (i) >= (n); --(i))
#define rep(i, n) REP(i, 0, n)
#define repn(i, n) REPN(i, 1, n)
#define repr(i, n) REPR(i, n, 0)
#define repnr(i, n) REPNR(i, n, 1)
#define all(s) (s).begin(), (s).end()
#define pb push_back
#define mp make_pair
#define fs first
#define sc second
template <typename T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> void co(const T n) { cout << n << endl; }
template <typename T> void cosp(const T n) { cout << n << ' '; }
void coVec(const Vec &v) {
  for (ll i : v)
    cosp(i);
  cout << endl;
}
ll in() {
  ll x;
  cin >> x;
  return x;
}
void sonic() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
}
void setp(const ll n) { cout << fixed << setprecision(n); }
const ll INF = 1e9 + 1;
const ll LINF = 1e18 + 1;
const ll MOD = 1e9 + 7;
// const ll MOD = 998244353;
const ld PI = acos(-1);
const ld EPS = 1e-11;

int main(void) {
  ll n;
  cin >> n;
  Vec a(n);
  rep(i, n) cin >> a[i];
  sort(all(a));
  reverse(all(a));

  ll ans = 0;
  VecB used(n, false);
  rep(i, n) {
    if (used[i])
      continue;
    else
      used[i] = true;
    ll b = 1;
    while (b <= a[i])
      b <<= 1;
    ll c = b - a[i];
    ll p = lower_bound(all(a), c, greater<>()) - a.begin();
    while (p != n && a[p] == c) {
      if (!used[p]) {
        ans++;
        used[p] = true;
        break;
      }
      p++;
    }
  }
  co(ans);

  return 0;
}


Fixed submission:
/*
confirm 0LL and 1LL
confirm cornercases such as 0
confirm times of cin < 10^6
*/
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using P = pair<ll, ll>;
using Pld = pair<ld, ld>;
using Vec = vector<ll>;
using VecP = vector<P>;
using VecB = vector<bool>;
using VecC = vector<char>;
using VecD = vector<ld>;
using VecS = vector<string>;
using VecVec = vector<Vec>;
using Graph = vector<VecP>;
template <typename T> using Vec1 = vector<T>;
template <typename T> using Vec2 = vector<Vec1<T>>;
#define REP(i, m, n) for (ll(i) = (m); (i) < (n); ++(i))
#define REPN(i, m, n) for (ll(i) = (m); (i) <= (n); ++(i))
#define REPR(i, m, n) for (ll(i) = (m)-1; (i) >= (n); --(i))
#define REPNR(i, m, n) for (ll(i) = (m); (i) >= (n); --(i))
#define rep(i, n) REP(i, 0, n)
#define repn(i, n) REPN(i, 1, n)
#define repr(i, n) REPR(i, n, 0)
#define repnr(i, n) REPNR(i, n, 1)
#define all(s) (s).begin(), (s).end()
#define pb push_back
#define mp make_pair
#define fs first
#define sc second
template <typename T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> void co(const T n) { cout << n << endl; }
template <typename T> void cosp(const T n) { cout << n << ' '; }
void coVec(const Vec &v) {
  for (ll i : v)
    cosp(i);
  cout << endl;
}
ll in() {
  ll x;
  cin >> x;
  return x;
}
void sonic() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
}
void setp(const ll n) { cout << fixed << setprecision(n); }
const ll INF = 1e9 + 1;
const ll LINF = 1e18 + 1;
const ll MOD = 1e9 + 7;
// const ll MOD = 998244353;
const ld PI = acos(-1);
const ld EPS = 1e-11;

int main(void) {
  ll n;
  cin >> n;
  Vec a(n);
  rep(i, n) cin >> a[i];
  sort(all(a));
  reverse(all(a));

  ll ans = 0;
  VecB used(n, false);
  rep(i, n) {
    if (used[i])
      continue;
    else
      used[i] = true;
    ll b = 1;
    while (b <= a[i])
      b <<= 1;
    ll c = b - a[i];
    if (i != n - 1 && a[i] == a[i + 1] && a[i + 1] == c) {
      ans++;
      used[i + 1] = true;
      continue;
    }
    ll p = lower_bound(all(a), c, greater<>()) - a.begin();
    while (p != n && a[p] == c) {
      if (!used[p]) {
        ans++;
        used[p] = true;
        break;
      }
      p++;
    }
  }
  co(ans);

  return 0;
}