Problem p04034 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <iostream>
#include <map>
#include <math.h>
#include <vector>
using namespace std;

using lli = long long int;
using Vint = vector<int>;
using Vlli = vector<lli>;
using Wint = vector<Vint>;
using Wlli = vector<Vlli>;
using pii = pair<int, int>;
using pll = pair<lli, lli>;

const int MOD = 1e9 + 7;
const int INFi = 2e9 + 1;
const lli INFl = (lli)(9e18) + 1;
const vector<pii> DXDY = {make_pair(1, 0), make_pair(-1, 0), make_pair(0, 1),
                          make_pair(0, -1)};
const char BR = '\n';

#define FOR(i, a, b) for (int(i) = (a); (i) < (b); (i)++)
#define FOReq(i, a, b) for (int(i) = (a); (i) <= (b); (i)++)
#define rFOR(i, a, b) for (int(i) = (b); (i) >= (a); i--)
#define FORstep(i, a, b, step) for (int(i) = (a); i < (b); i += (step))
#define REP(i, n) FOR(i, 0, n)
#define rREP(i, n) rFOR(i, 0, (n - 1))
#define vREP(i, vec) for (auto &(i) : (vec))

template <class T> inline int argmin(vector<T> vec) {
  return min_element(vec.begin(), vec.end()) - vec.begin();
}
template <class T> inline int argmax(vector<T> vec) {
  return max_element(vec.begin(), vec.end()) - vec.begin();
}
template <class T> inline void chmax(T &a, T b) { a = max(a, b); }
template <class T> inline void chmin(T &a, T b) { a = min(a, b); }
inline int toInt(string &s) {
  int res = 0;
  for (char a : s)
    res = 10 * res + (a - '0');
  return res;
}
inline int toInt(const string s) {
  int res = 0;
  for (char a : s)
    res = 10 * res + (a - '0');
  return res;
}
inline long long int toLong(string &s) {
  lli res = 0;
  for (char a : s)
    res = 10 * res + (a - '0');
  return res;
}
inline long long int toLong(const string s) {
  lli res = 0;
  for (char a : s)
    res = 10 * res + (a - '0');
  return res;
}
template <class T> inline std::string toString(T n) {
  if (n == 0)
    return "0";
  std::string res = "";
  if (n < 0) {
    n = -n;
    while (n != 0) {
      res += (char)(n % 10 + '0');
      n /= 10;
    }
    std::reverse(res.begin(), res.end());
    return '-' + res;
  }
  while (n != 0) {
    res += (char)(n % 10 + '0');
    n /= 10;
  }
  std::reverse(res.begin(), res.end());
  return res;
}

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

int main(void) {
  int n, m;
  cin >> n >> m;
  Vint number_balls(n, 1);
  vector<bool> Pexit(n, false);
  Pexit.at(0) = true;
  int x, y;
  REP(i, m) {
    cin >> x >> y;
    x--;
    y--;
    number_balls.at(x)--;
    number_balls.at(y)++;
    if (Pexit.at(i)) {
      Pexit.at(y) = true;
      if (number_balls.at(x) == 0) {
        Pexit.at(x) = false;
      }
    }
  }
  cout << std::count(Pexit.begin(), Pexit.end(), true) << BR;
  return 0;
}

Fixed submission:
#include <algorithm>
#include <iostream>
#include <map>
#include <math.h>
#include <vector>
using namespace std;

using lli = long long int;
using Vint = vector<int>;
using Vlli = vector<lli>;
using Wint = vector<Vint>;
using Wlli = vector<Vlli>;
using pii = pair<int, int>;
using pll = pair<lli, lli>;

const int MOD = 1e9 + 7;
const int INFi = 2e9 + 1;
const lli INFl = (lli)(9e18) + 1;
const vector<pii> DXDY = {make_pair(1, 0), make_pair(-1, 0), make_pair(0, 1),
                          make_pair(0, -1)};
const char BR = '\n';

#define FOR(i, a, b) for (int(i) = (a); (i) < (b); (i)++)
#define FOReq(i, a, b) for (int(i) = (a); (i) <= (b); (i)++)
#define rFOR(i, a, b) for (int(i) = (b); (i) >= (a); i--)
#define FORstep(i, a, b, step) for (int(i) = (a); i < (b); i += (step))
#define REP(i, n) FOR(i, 0, n)
#define rREP(i, n) rFOR(i, 0, (n - 1))
#define vREP(i, vec) for (auto &(i) : (vec))

template <class T> inline int argmin(vector<T> vec) {
  return min_element(vec.begin(), vec.end()) - vec.begin();
}
template <class T> inline int argmax(vector<T> vec) {
  return max_element(vec.begin(), vec.end()) - vec.begin();
}
template <class T> inline void chmax(T &a, T b) { a = max(a, b); }
template <class T> inline void chmin(T &a, T b) { a = min(a, b); }
inline int toInt(string &s) {
  int res = 0;
  for (char a : s)
    res = 10 * res + (a - '0');
  return res;
}
inline int toInt(const string s) {
  int res = 0;
  for (char a : s)
    res = 10 * res + (a - '0');
  return res;
}
inline long long int toLong(string &s) {
  lli res = 0;
  for (char a : s)
    res = 10 * res + (a - '0');
  return res;
}
inline long long int toLong(const string s) {
  lli res = 0;
  for (char a : s)
    res = 10 * res + (a - '0');
  return res;
}
template <class T> inline std::string toString(T n) {
  if (n == 0)
    return "0";
  std::string res = "";
  if (n < 0) {
    n = -n;
    while (n != 0) {
      res += (char)(n % 10 + '0');
      n /= 10;
    }
    std::reverse(res.begin(), res.end());
    return '-' + res;
  }
  while (n != 0) {
    res += (char)(n % 10 + '0');
    n /= 10;
  }
  std::reverse(res.begin(), res.end());
  return res;
}

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

int main(void) {
  int n, m;
  cin >> n >> m;
  Vint number_balls(n, 1);
  vector<bool> Pexit(n, false);
  Pexit.at(0) = true;
  int x, y;
  REP(i, m) {
    cin >> x >> y;
    x--;
    y--;
    number_balls.at(x)--;
    number_balls.at(y)++;
    if (Pexit.at(x)) {
      Pexit.at(y) = true;
      if (number_balls.at(x) == 0) {
        Pexit.at(x) = false;
      }
    }
  }
  cout << std::count(Pexit.begin(), Pexit.end(), true) << BR;
  return 0;
}