Problem p02597 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
const int INF = 1e9;
const int MOD = 1e9 + 7;
const long long LINF = 1e18;
#define dump(x) cout << 'x' << ' = ' << (x) << ` `;
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define REPR(i, n) for (int i = n; i >= 0; i--)
#define FOREACH(x, a) for (auto &(x) : (a))
typedef long long ll;
using namespace std;
typedef pair<ll, ll> P;

template <typename T> void print(const vector<T> &x) {
  int n = x.size();
  rep(i, n) {
    cout << x[i];
    if (i != n - 1)
      cout << " ";
    else
      cout << endl;
  }
}

template <typename T> void print(const vector<vector<T>> &x) {
  int n = x.size();
  rep(i, n) {
    rep(j, x[i].size()) { cout << x[i][j] << " "; }
    cout << endl;
  }
}

template <typename T> void print(const vector<T> &x, int n) {
  rep(i, n) {
    cout << x[i];
    if (i != n - 1)
      cout << " ";
    else
      cout << endl;
  }
}

template <typename T> void print(const vector<vector<T>> &x, int n, int m) {
  rep(i, n) {
    rep(j, m) { cout << x[i][j] << " "; }
    cout << endl;
  }
}

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

void input_init() {
  cin.tie(0);
  ios::sync_with_stdio(false);
}

int main(int argc, char const *argv[]) {
  int n;
  cin >> n;
  string s;
  cin >> s;

  int ans = 0;
  vector<int> idx;
  for (int i = n - 1; i >= 0; --i) {
    if (s[i] == 'R')
      idx.push_back(i);
  }
  reverse(idx.begin(), idx.end());

  rep(i, n) {
    if (s[i] == 'W') {
      if (i < idx.back()) {
        idx.pop_back();
        ans++;
      }
    }
  }

  cout << ans << '\n';
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
const int INF = 1e9;
const int MOD = 1e9 + 7;
const long long LINF = 1e18;
#define dump(x) cout << 'x' << ' = ' << (x) << ` `;
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define REPR(i, n) for (int i = n; i >= 0; i--)
#define FOREACH(x, a) for (auto &(x) : (a))
typedef long long ll;
using namespace std;
typedef pair<ll, ll> P;

template <typename T> void print(const vector<T> &x) {
  int n = x.size();
  rep(i, n) {
    cout << x[i];
    if (i != n - 1)
      cout << " ";
    else
      cout << endl;
  }
}

template <typename T> void print(const vector<vector<T>> &x) {
  int n = x.size();
  rep(i, n) {
    rep(j, x[i].size()) { cout << x[i][j] << " "; }
    cout << endl;
  }
}

template <typename T> void print(const vector<T> &x, int n) {
  rep(i, n) {
    cout << x[i];
    if (i != n - 1)
      cout << " ";
    else
      cout << endl;
  }
}

template <typename T> void print(const vector<vector<T>> &x, int n, int m) {
  rep(i, n) {
    rep(j, m) { cout << x[i][j] << " "; }
    cout << endl;
  }
}

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

void input_init() {
  cin.tie(0);
  ios::sync_with_stdio(false);
}

int main(int argc, char const *argv[]) {
  int n;
  cin >> n;
  string s;
  cin >> s;

  int ans = 0;
  vector<int> idx;
  for (int i = n - 1; i >= 0; --i) {
    if (s[i] == 'R')
      idx.push_back(i);
  }
  reverse(idx.begin(), idx.end());

  if (idx.empty()) {
    cout << 0 << '\n';
    return 0;
  }

  rep(i, n) {
    if (s[i] == 'W') {
      if (i < idx.back()) {
        idx.pop_back();
        ans++;
      }
    }
  }

  cout << ans << '\n';
  return 0;
}