Problem p03201 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <bitset>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>
#define FAST ios::sync_with_stdio(false)
#define abs(a) ((a) >= 0 ? (a) : -(a))
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define mem(a, b) memset(a, b, sizeof(a))
#define max(a, b) ((a) > (b) ? (a) : (b))
#define min(a, b) ((a) < (b) ? (a) : (b))
#define rep(i, a, n) for (int i = a; i <= n; ++i)
#define per(i, n, a) for (int i = n; i >= a; --i)
#define pb push_back
#define mp make_pair
#define fi first
#define se second
using namespace std;
typedef long long ll;
typedef pair<ll, ll> PII;
const int maxn = 1e6 + 200;
const int inf = 0x3f3f3f3f;
const double eps = 1e-7;
const double pi = acos(-1.0);
const int mod = 1e9 + 7;
inline int lowbit(int x) { return x & (-x); }
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
void ex_gcd(ll a, ll b, ll &d, ll &x, ll &y) {
  if (!b) {
    d = a, x = 1, y = 0;
  } else {
    ex_gcd(b, a % b, d, y, x);
    y -= x * (a / b);
  }
} // x=(x%(b/d)+(b/d))%(b/d);
inline ll qpow(ll a, ll b, ll MOD = mod) {
  ll res = 1;
  a %= MOD;
  while (b > 0) {
    if (b & 1)
      res = res * a % MOD;
    a = a * a % MOD;
    b >>= 1;
  }
  return res;
}
inline ll inv(ll x, ll p) { return qpow(x, p - 2, p); }
inline ll Jos(ll n, ll k, ll s = 1) {
  ll res = 0;
  rep(i, 1, n + 1) res = (res + k) % i;
  return (res + s) % n;
}
inline ll read() {
  ll f = 1;
  ll x = 0;
  char ch = getchar();
  while (ch > '9' || ch < '0') {
    if (ch == '-')
      f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9')
    x = (x << 3) + (x << 1) + ch - '0', ch = getchar();
  return x * f;
}
int dir[4][2] = {{1, 0}, {-1, 0}, {0, 1}, {0, -1}};

unordered_map<ll, ll> Map;
ll a[maxn];

int main() {
  ll n = read();
  rep(i, 1, n) a[i] = read(), Map[a[i]]++;
  sort(a + 1, a + 1 + n);
  ll ans = 0;
  per(i, n, 1) {
    for (ll j = 0; (1 << j) <= 1e10; j++) {
      if ((1 << j) < a[i])
        continue;
      if (Map[(1 << j) - a[i]] >= 1 &&
          ((1 << j) - a[i] == a[i] ? Map[a[i]] >= 2 : Map[a[i]] >= 1)) {
        Map[a[i]]--, Map[(1 << j) - a[i]]--, ans++;
        break;
      }
    }
  }
  cout << ans << '\n';
  return 0;
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>
#define FAST ios::sync_with_stdio(false)
#define abs(a) ((a) >= 0 ? (a) : -(a))
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define mem(a, b) memset(a, b, sizeof(a))
#define max(a, b) ((a) > (b) ? (a) : (b))
#define min(a, b) ((a) < (b) ? (a) : (b))
#define rep(i, a, n) for (int i = a; i <= n; ++i)
#define per(i, n, a) for (int i = n; i >= a; --i)
#define pb push_back
#define mp make_pair
#define fi first
#define se second
using namespace std;
typedef long long ll;
typedef pair<ll, ll> PII;
const int maxn = 1e6 + 200;
const int inf = 0x3f3f3f3f;
const double eps = 1e-7;
const double pi = acos(-1.0);
const int mod = 1e9 + 7;
inline int lowbit(int x) { return x & (-x); }
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
void ex_gcd(ll a, ll b, ll &d, ll &x, ll &y) {
  if (!b) {
    d = a, x = 1, y = 0;
  } else {
    ex_gcd(b, a % b, d, y, x);
    y -= x * (a / b);
  }
} // x=(x%(b/d)+(b/d))%(b/d);
inline ll qpow(ll a, ll b, ll MOD = mod) {
  ll res = 1;
  a %= MOD;
  while (b > 0) {
    if (b & 1)
      res = res * a % MOD;
    a = a * a % MOD;
    b >>= 1;
  }
  return res;
}
inline ll inv(ll x, ll p) { return qpow(x, p - 2, p); }
inline ll Jos(ll n, ll k, ll s = 1) {
  ll res = 0;
  rep(i, 1, n + 1) res = (res + k) % i;
  return (res + s) % n;
}
inline ll read() {
  ll f = 1;
  ll x = 0;
  char ch = getchar();
  while (ch > '9' || ch < '0') {
    if (ch == '-')
      f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9')
    x = (x << 3) + (x << 1) + ch - '0', ch = getchar();
  return x * f;
}
int dir[4][2] = {{1, 0}, {-1, 0}, {0, 1}, {0, -1}};

unordered_map<ll, ll> Map;
ll a[maxn];

int main() {
  ll n = read();
  rep(i, 1, n) a[i] = read(), Map[a[i]]++;
  sort(a + 1, a + 1 + n);
  ll ans = 0;
  per(i, n, 1) {
    for (ll j = 0; (1LL << j) <= 1e10; j++) {
      if ((1 << j) < a[i])
        continue;
      if (Map[(1 << j) - a[i]] >= 1 &&
          ((1 << j) - a[i] == a[i] ? Map[a[i]] >= 2 : Map[a[i]] >= 1)) {
        Map[a[i]]--, Map[(1 << j) - a[i]]--, ans++;
        break;
      }
    }
  }
  cout << ans << '\n';
  return 0;
}