Problem p02597 (C++)

Buggy submission (status: Runtime Error):
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
#define speedX                                                                 \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(NULL);                                                               \
  cout.tie(NULL)
using namespace std;
// #define ONLINE_JUDGE
#ifndef ONLINE_JUDGE
template <typename T> void __p(T a) { cout << a << " "; }
template <typename T, typename F> void __p(pair<T, F> a) {
  cout << "{ ";
  __p(a.first);
  __p(a.second);
  cout << "}";
}
template <typename T> void __f(const char *s, T t) {
  cout << s << " : ";
  __p(t);
}
template <typename T> void __t(const char *s, const T &x) {
  cout << s << " : { ";
  for (auto it : x)
    __p(it);
  cout << "} ";
}
template <typename T, typename... F> void __f(const char *s, T t, F... f) {
  int i = 0;
  for (;; ++i)
    if (s[i] == ',')
      break;
  cout.write(s, i) << " : ";
  __p(t);
  cout << " | ";
  __f(s + i + 1, f...);
}
#define trace(...)                                                             \
  {                                                                            \
    cout << "LINE: " << __LINE__ << " || ";                                    \
    __f(#__VA_ARGS__, __VA_ARGS__);                                            \
    cout << "\n\n";                                                            \
  }
#define debug(...)                                                             \
  {                                                                            \
    cout << "LINE: " << __LINE__ << " || ";                                    \
    __t(#__VA_ARGS__, __VA_ARGS__);                                            \
    cout << "\n\n";                                                            \
  }
int begtime = clock();
#define end_time()                                                             \
  cout << "\n\nTime of execution: "                                            \
       << (clock() - begtime) * 1000 / CLOCKS_PER_SEC << " ms\n\n";
#else
#define trace(...)
#define debug(...)
#define end_time()
#endif

#define int long long
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<pii, int> piii;
typedef vector<int> vi;
typedef vector<pii> vii;

inline bool equals(ld a, ld b) { return fabs(a - b) < 1e-9; }
inline int gcd(int a, int b) {
  a = abs(a);
  b = abs(b);
  while (a > 0 && b > 0)
    (a > b ? a %= b : b %= a);
  return (a == 0 ? b : a);
}
inline int power(int x, int n, int m = LLONG_MAX) {
  int res = 1;
  x = (x % m + m) % m;
  while (n) {
    if (n & 1)
      res = (res * x) % m;
    x = (x * x) % m;
    n >>= 1;
  }
  return res;
}
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define ff first
#define ss second
#define uset unordered_set
#define umap unordered_map
#define all(x) x.begin(), x.end()
#define mod 1000000007

void solve() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  vi w, r;
  for (int i = 0; i < n; i++) {
    if (s[i] == 'W')
      w.pb(i);
    else
      r.pb(i);
  }
  int ans = 0;
  for (auto x : w) {
    int d = r[r.size() - 1];
    if (d > x) {
      r.pop_back();
      ans++;
    } else
      break;
  }
  cout << ans << "\n";
}
signed main() {
  speedX;
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
  int t = 1;
  // cin>>t;
  for (int i = 1; i <= t; ++i) {
    // cout<<"Case #"<<i<<": ";
    solve();
  }
  end_time();
  return 0;
}


Fixed submission:
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
#define speedX                                                                 \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(NULL);                                                               \
  cout.tie(NULL)
using namespace std;
// #define ONLINE_JUDGE
#ifndef ONLINE_JUDGE
template <typename T> void __p(T a) { cout << a << " "; }
template <typename T, typename F> void __p(pair<T, F> a) {
  cout << "{ ";
  __p(a.first);
  __p(a.second);
  cout << "}";
}
template <typename T> void __f(const char *s, T t) {
  cout << s << " : ";
  __p(t);
}
template <typename T> void __t(const char *s, const T &x) {
  cout << s << " : { ";
  for (auto it : x)
    __p(it);
  cout << "} ";
}
template <typename T, typename... F> void __f(const char *s, T t, F... f) {
  int i = 0;
  for (;; ++i)
    if (s[i] == ',')
      break;
  cout.write(s, i) << " : ";
  __p(t);
  cout << " | ";
  __f(s + i + 1, f...);
}
#define trace(...)                                                             \
  {                                                                            \
    cout << "LINE: " << __LINE__ << " || ";                                    \
    __f(#__VA_ARGS__, __VA_ARGS__);                                            \
    cout << "\n\n";                                                            \
  }
#define debug(...)                                                             \
  {                                                                            \
    cout << "LINE: " << __LINE__ << " || ";                                    \
    __t(#__VA_ARGS__, __VA_ARGS__);                                            \
    cout << "\n\n";                                                            \
  }
int begtime = clock();
#define end_time()                                                             \
  cout << "\n\nTime of execution: "                                            \
       << (clock() - begtime) * 1000 / CLOCKS_PER_SEC << " ms\n\n";
#else
#define trace(...)
#define debug(...)
#define end_time()
#endif

#define int long long
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<pii, int> piii;
typedef vector<int> vi;
typedef vector<pii> vii;

inline bool equals(ld a, ld b) { return fabs(a - b) < 1e-9; }
inline int gcd(int a, int b) {
  a = abs(a);
  b = abs(b);
  while (a > 0 && b > 0)
    (a > b ? a %= b : b %= a);
  return (a == 0 ? b : a);
}
inline int power(int x, int n, int m = LLONG_MAX) {
  int res = 1;
  x = (x % m + m) % m;
  while (n) {
    if (n & 1)
      res = (res * x) % m;
    x = (x * x) % m;
    n >>= 1;
  }
  return res;
}
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define ff first
#define ss second
#define uset unordered_set
#define umap unordered_map
#define all(x) x.begin(), x.end()
#define mod 1000000007

void solve() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  vi w, r;
  for (int i = 0; i < n; i++) {
    if (s[i] == 'W')
      w.pb(i);
    else
      r.pb(i);
  }
  int ans = 0;
  for (auto x : w) {
    if (r.size() == 0)
      break;
    int d = r[r.size() - 1];
    if (d > x) {
      r.pop_back();
      ans++;
    } else
      break;
  }
  cout << ans << "\n";
}
signed main() {
  speedX;
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
  int t = 1;
  // cin>>t;
  for (int i = 1; i <= t; ++i) {
    // cout<<"Case #"<<i<<": ";
    solve();
  }
  end_time();
  return 0;
}