Problem p02641 (C++) — buggy submission, verdict: Runtime Error

#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define vecll vector<long long>
#define vec2ll vector<vector<long long>>
#define vecpairll vector<pair<long long, long long>>
#define forll(s, a, b) for (long long s = a; s < b; s++)
#define forllde(s, a) for (long long s = a; s > 0; s--)

template <typename A, typename B> string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string &s) { return '"' + s + '"'; }

string to_string(const char *s) { return to_string((string)s); }

string to_string(bool b) { return (b ? "true" : "false"); }

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}

template <size_t N> string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
#define debug(...) cerr << "<" << #__VA_ARGS__ << ">:", debug_out(__VA_ARGS__)
#define INF LLONG_MAX

int main() {
  ll x, n, a, min_v = INF;
  cin >> x >> n;
  vecll p(101, 0);

  forll(i, 0, n) {
    cin >> a;
    p[a] = 1;
  }

  forll(i, x, 101) {
    if (p[i] == 0) {
      min_v = i;
      break;
    }
  }

  if (min_v == INF) {
    min_v = 101;
  }

  for (ll i = x; i >= 0; i--) {
    if (p[i] == 0) {
      if ((min_v - x) >= x - i)
        min_v = i;
      break;
    }
  }

  cout << min_v << endl;
}


Error: 0

Fixed submission:

#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define vecll vector<long long>
#define vec2ll vector<vector<long long>>
#define vecpairll vector<pair<long long, long long>>
#define forll(s, a, b) for (long long s = a; s < b; s++)
#define forllde(s, a) for (long long s = a; s > 0; s--)

template <typename A, typename B> string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string &s) { return '"' + s + '"'; }

string to_string(const char *s) { return to_string((string)s); }

string to_string(bool b) { return (b ? "true" : "false"); }

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}

template <size_t N> string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
#define debug(...) cerr << "<" << #__VA_ARGS__ << ">:", debug_out(__VA_ARGS__)
#define INF LLONG_MAX

int main() {
  ll x, n, a, min_v = 102;
  cin >> x >> n;
  vecll p(101, 0);
  forll(i, 0, n) {
    cin >> a;
    p[a] = 1;
  }
  forll(i, x, 102) {
    if (p[i] == 0) {
      min_v = i;
      break;
    }
  }
  for (ll i = x; i >= 0; i--)
    if (p[i] == 0) {
      if ((min_v - x) >= x - i)
        min_v = i;
      break;
    }
  cout << min_v << endl;
}