Problem p03201 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#include <sys/types.h>
#include <unistd.h>

#define _overload(_1, _2, _3, name, ...) name
#define _rep(i, n) _range(i, 0, n)
#define _range(i, a, b) for (int i = int(a); i < int(b); ++i)
#define rep(...) _overload(__VA_ARGS__, _range, _rep, )(__VA_ARGS__)

#define _rrep(i, n) _rrange(i, n, 0)
#define _rrange(i, a, b) for (int i = int(a) - 1; i >= int(b); --i)
#define rrep(...) _overload(__VA_ARGS__, _rrange, _rrep, )(__VA_ARGS__)

#define _all(arg) begin(arg), end(arg)
#define uniq(arg) sort(_all(arg)), (arg).erase(unique(_all(arg)), end(arg))
#define getidx(ary, key) lower_bound(_all(ary), key) - begin(ary)
#define clr(a, b) memset((a), (b), sizeof(a))
#define bit(n) (1LL << (n))
#define popcount(n) (__builtin_popcountll(n))

using namespace std;

template <class T> bool chmax(T &a, const T &b) {
  return (a < b) ? (a = b, 1) : 0;
}
template <class T> bool chmin(T &a, const T &b) {
  return (b < a) ? (a = b, 1) : 0;
}

using ll = long long;
using R = long double;
const R EPS = 1e-9L; // [-1000,1000]->EPS=1e-8 [-10000,10000]->EPS=1e-7
inline int sgn(const R &r) { return (r > EPS) - (r < -EPS); }
inline R sq(R x) { return sqrt(max(x, 0.0L)); }

const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};

const pid_t pid = getpid();
// Problem Specific Parameter:

map<ll, int> cnt;

int main(void) {
  int n;
  cin >> n;
  vector<ll> a(n);
  rep(i, n) cin >> a[i];
  rep(i, n) cnt[a[i]]++;

  int ans = 0;
  rrep(k, 31) {
    const ll tar = (1LL << k);
    for (auto &it : cnt) {
      const ll val = it.first;
      const ll nval = tar - it.first;
      if (val < nval)
        continue;

      if (val == nval) {
        const int num = it.second / 2;
        ans += num;
        it.second -= 2 * num;
      } else {
        const int num = min(it.second, cnt[nval]);
        ans += num;
        it.second -= num;
        cnt[nval] -= num;
      }
    }
  }

  cout << ans << endl;

  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
#include <sys/types.h>
#include <unistd.h>

#define _overload(_1, _2, _3, name, ...) name
#define _rep(i, n) _range(i, 0, n)
#define _range(i, a, b) for (int i = int(a); i < int(b); ++i)
#define rep(...) _overload(__VA_ARGS__, _range, _rep, )(__VA_ARGS__)

#define _rrep(i, n) _rrange(i, n, 0)
#define _rrange(i, a, b) for (int i = int(a) - 1; i >= int(b); --i)
#define rrep(...) _overload(__VA_ARGS__, _rrange, _rrep, )(__VA_ARGS__)

#define _all(arg) begin(arg), end(arg)
#define uniq(arg) sort(_all(arg)), (arg).erase(unique(_all(arg)), end(arg))
#define getidx(ary, key) lower_bound(_all(ary), key) - begin(ary)
#define clr(a, b) memset((a), (b), sizeof(a))
#define bit(n) (1LL << (n))
#define popcount(n) (__builtin_popcountll(n))

using namespace std;

template <class T> bool chmax(T &a, const T &b) {
  return (a < b) ? (a = b, 1) : 0;
}
template <class T> bool chmin(T &a, const T &b) {
  return (b < a) ? (a = b, 1) : 0;
}

using ll = long long;
using R = long double;
const R EPS = 1e-9L; // [-1000,1000]->EPS=1e-8 [-10000,10000]->EPS=1e-7
inline int sgn(const R &r) { return (r > EPS) - (r < -EPS); }
inline R sq(R x) { return sqrt(max(x, 0.0L)); }

const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};

const pid_t pid = getpid();
// Problem Specific Parameter:

map<ll, int> cnt;

int main(void) {
  int n;
  cin >> n;
  vector<ll> a(n);
  rep(i, n) cin >> a[i];
  rep(i, n) cnt[a[i]]++;

  int ans = 0;
  rrep(k, 31) {
    const ll tar = (1LL << k);
    for (auto &it : cnt) {
      const ll val = it.first;
      const ll nval = tar - it.first;
      if (val < tar / 2 or tar <= val)
        continue;

      if (val == nval) {
        const int num = it.second / 2;
        ans += num;
        it.second -= 2 * num;
      } else {
        const int num = min(it.second, cnt[nval]);
        ans += num;
        it.second -= num;
        cnt[nval] -= num;
      }
    }
  }

  cout << ans << endl;

  return 0;
}