Problem p03818 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
/*{{{*/ // template
#define REP(i, n) for (int i = 0; i < n; i++)
#define rep(i, n) for (int i = 0; i < n; i++)
#define INF 1 << 29
#define LINF LLONG_MAX / 3
#define MP make_pair
#define PB push_back
#define pb push_back
#define EB emplace_back
#define ALL(v) (v).begin(), (v).end()
#define all(v) ALL(v)
#define sz(x) (int)(x).size()
#define debug(x) cerr << #x << ":" << x << endl
#define debug2(x, y) cerr << #x << "," << #y ":" << x << "," << y << endl
// struct fin{ fin(){ cin.tie(0); ios::sync_with_stdio(false); } } fin_;
struct Double {
  double d;
  explicit Double(double x) : d(x) {}
};
ostream &operator<<(ostream &os, const Double x) {
  os << fixed << setprecision(20) << x.d;
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "[";
  for (const auto &v : vec) {
    os << v << ",";
  }
  os << "]";
  return os;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << "(" << p.first << "," << p.second << ")";
  return os;
}
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
constexpr double eps = 1e-14;
constexpr ll mod = 1e9 + 7;
const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
/*}}}*/

#define MAX_A 10000

int N;

int main() {
  cin >> N;
  vector<int> A(N);
  rep(i, N) {
    cin >> A[i];
    A[i]--;
  }
  int ma = *max_element(all(A));
  vector<int> cnt(ma + 1, 0);
  rep(i, N) cnt[A[i]]++;
  for (int i = 0; i <= ma; i++)
    if (cnt[i] > 2) {
      if (cnt[i] % 2 == 1)
        cnt[i] = 1;
      else
        cnt[i] = 2;
    }

  vector<int> v;
  for (int i = 0; i <= ma; i++)
    if (cnt[i] == 2)
      v.push_back(i);

  for (int i = 0; i < sz(v) - 1; i++) {
    if (cnt[v[i]] < 2)
      continue;
    cnt[v[i]]--;
    cnt[v[i + 1]]--;
  }

  ll sum = 0;
  for (int i = 0; i <= ma; i++)
    sum += cnt[i];
  if (cnt[v[sz(v) - 1]] == 2)
    sum -= 2;
  cout << sum << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
/*{{{*/ // template
#define REP(i, n) for (int i = 0; i < n; i++)
#define rep(i, n) for (int i = 0; i < n; i++)
#define INF 1 << 29
#define LINF LLONG_MAX / 3
#define MP make_pair
#define PB push_back
#define pb push_back
#define EB emplace_back
#define ALL(v) (v).begin(), (v).end()
#define all(v) ALL(v)
#define sz(x) (int)(x).size()
#define debug(x) cerr << #x << ":" << x << endl
#define debug2(x, y) cerr << #x << "," << #y ":" << x << "," << y << endl
// struct fin{ fin(){ cin.tie(0); ios::sync_with_stdio(false); } } fin_;
struct Double {
  double d;
  explicit Double(double x) : d(x) {}
};
ostream &operator<<(ostream &os, const Double x) {
  os << fixed << setprecision(20) << x.d;
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "[";
  for (const auto &v : vec) {
    os << v << ",";
  }
  os << "]";
  return os;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << "(" << p.first << "," << p.second << ")";
  return os;
}
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
constexpr double eps = 1e-14;
constexpr ll mod = 1e9 + 7;
const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
/*}}}*/

#define MAX_A 10000

int N;

int main() {
  cin >> N;
  vector<int> A(N);
  rep(i, N) {
    cin >> A[i];
    A[i]--;
  }
  int ma = *max_element(all(A));
  vector<int> cnt(ma + 1, 0);
  rep(i, N) cnt[A[i]]++;
  for (int i = 0; i <= ma; i++)
    if (cnt[i] > 2) {
      if (cnt[i] % 2 == 1)
        cnt[i] = 1;
      else
        cnt[i] = 2;
    }

  vector<int> v;
  for (int i = 0; i <= ma; i++)
    if (cnt[i] == 2)
      v.push_back(i);
  if (v.size() == 0) {
    ll sum = accumulate(cnt.begin(), cnt.end(), 0ll);
    cout << sum << endl;
    return 0;
  }

  for (int i = 0; i < sz(v) - 1; i++) {
    if (cnt[v[i]] < 2)
      continue;
    cnt[v[i]]--;
    cnt[v[i + 1]]--;
  }

  ll sum = 0;
  for (int i = 0; i <= ma; i++)
    sum += cnt[i];
  if (cnt[v[sz(v) - 1]] == 2)
    sum -= 2;
  cout << sum << endl;
}