Problem p02597 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ull = unsigned long long;
using ld = long double;

#define MOD 1000000007LL

#define rep(i, n) for (ll(i) = 0LL; (i) < (ll)(n); (i)++)
#define rep2(i, s, e) for (ll(i) = (ll)(s); (i) < (ll)(e); (i)++)
#define repi(i, n) for (ll(i) = 0LL; (i) <= (ll)(n); (i)++)
#define repi2(i, s, e) for (ll(i) = (ll)(s); (i) <= (ll)(e); (i)++)
#define per(i, n) for (ll(i) = (ll)(n)-1LL; (i) >= 0LL; (i)--)
#define per2(i, s, e) for (ll(i) = (ll)(s)-1LL; (i) >= (ll)(e); (i)--)
#define peri(i, n) for (ll(i) = (ll)(n); (i) >= 0LL; (i)--)
#define peri2(i, s, e) for (ll(i) = (ll)(s); (i) >= (ll)(e); (i)--)
#define iter(i, it) for (auto &(i) : (it))

template <typename T, typename U>
ostream &operator<<(ostream &s, const pair<T, U> m) {
  cout << "(" << m.first << ", " << m.second << ")";
  return s;
}
template <typename T, typename U>
ostream &operator<<(ostream &s, const map<T, U> m) {
  ll c = 0;
  cout << "{ ";
  iter(i, m) cout << i << (c++ == m.size() - 1 ? " " : ", ");
  cout << "}";
  return s;
}
template <typename T> ostream &operator<<(ostream &s, const vector<T> &v) {
  cout << "{ ";
  rep(i, v.size()) cout << v[i] << (i == v.size() - 1 ? " " : ", ");
  cout << "}";
  return s;
}
template <typename T> ostream &operator<<(ostream &s, const list<T> &v) {
  ll c = 0;
  cout << "{ ";
  iter(i, v) cout << i << (c++ == v.size() - 1 ? " " : ", ");
  cout << "}";
  return s;
}

int main(void) {
  ll N;
  string C;
  cin >> N >> C;
  vector<ll> R;
  ll ans = 0;
  rep(i, C.size()) {
    if (C[i] == 'R')
      R.push_back(i);
  }
  rep(i, C.size()) {
    if (C[i] == 'W' && i < R.back())
      ans++, R.pop_back();
  }
  cout << ans << endl;
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ull = unsigned long long;
using ld = long double;

#define MOD 1000000007LL

#define rep(i, n) for (ll(i) = 0LL; (i) < (ll)(n); (i)++)
#define rep2(i, s, e) for (ll(i) = (ll)(s); (i) < (ll)(e); (i)++)
#define repi(i, n) for (ll(i) = 0LL; (i) <= (ll)(n); (i)++)
#define repi2(i, s, e) for (ll(i) = (ll)(s); (i) <= (ll)(e); (i)++)
#define per(i, n) for (ll(i) = (ll)(n)-1LL; (i) >= 0LL; (i)--)
#define per2(i, s, e) for (ll(i) = (ll)(s)-1LL; (i) >= (ll)(e); (i)--)
#define peri(i, n) for (ll(i) = (ll)(n); (i) >= 0LL; (i)--)
#define peri2(i, s, e) for (ll(i) = (ll)(s); (i) >= (ll)(e); (i)--)
#define iter(i, it) for (auto &(i) : (it))

template <typename T, typename U>
ostream &operator<<(ostream &s, const pair<T, U> m) {
  cout << "(" << m.first << ", " << m.second << ")";
  return s;
}
template <typename T, typename U>
ostream &operator<<(ostream &s, const map<T, U> m) {
  ll c = 0;
  cout << "{ ";
  iter(i, m) cout << i << (c++ == m.size() - 1 ? " " : ", ");
  cout << "}";
  return s;
}
template <typename T> ostream &operator<<(ostream &s, const vector<T> &v) {
  cout << "{ ";
  rep(i, v.size()) cout << v[i] << (i == v.size() - 1 ? " " : ", ");
  cout << "}";
  return s;
}
template <typename T> ostream &operator<<(ostream &s, const list<T> &v) {
  ll c = 0;
  cout << "{ ";
  iter(i, v) cout << i << (c++ == v.size() - 1 ? " " : ", ");
  cout << "}";
  return s;
}

int main(void) {
  ll N;
  string C;
  cin >> N >> C;
  vector<ll> R;
  ll ans = 0;
  rep(i, C.size()) {
    if (C[i] == 'R')
      R.push_back(i);
  }
  rep(i, C.size()) {
    if (C[i] == 'W' && !R.empty() && i < R.back())
      ans++, R.pop_back();
  }
  cout << ans << endl;
  return 0;
}