Problem p02597 (C++) — buggy submission, verdict: Runtime Error

#include <algorithm>
#include <assert.h>
#include <bitset>
#include <cmath>
#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>

using namespace std;
using lint = int64_t;

#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep1(i, n) for (lint i = 1; i < (n); ++i)
#define repn(i, a, b) for (lint i = (a); i < (b); ++i)
#define rep_inv(i, n) for (lint i = (n); i >= 0; --i)
#define all(vec) vec.begin(), vec.end()
#define cend printf("\\n")

// constexpr lint mod = 998'244'353LL;
constexpr lint Mod = 1000'000'007LL;
constexpr lint Inf = 3'000'000'000'000'000'007LL; // 3e18+7
constexpr double Pi = 3.141592653589793;

template <class T> using prique = priority_queue<T>;
template <class T> using prique_inv = priority_queue<T, vector<T>, greater<T>>;
template <class T, class U>
inline istream &operator>>(istream &is, pair<T, U> &rhs) {
  return is >> rhs.first >> rhs.second;
}
template <class T, class U>
inline ostream &operator<<(ostream &os, const pair<T, U> &rhs) {
  return os << rhs.first << " " << rhs.second;
}
template <class InputIterator>
void arrin(InputIterator first, InputIterator last) {
  for (; first != last; ++first)
    cin >> (*first);
}
template <class InputIterator>
void arrout(InputIterator first, InputIterator last) {
  for (; first != last; ++first) {
    cout << (*first) << ((first + 1) != last ? " " : "\\n");
  }
}

bool pri(lint x) {
  for (lint i = 2; i * i <= x; ++i) {
    if (x % i == 0)
      return false;
  }
  return 1 < x;
}

lint fact[3000000];
void fact_init(lint n, lint m = Mod) {
  if (3000000 <= n)
    return;
  fact[0] = fact[1] = 1;
  for (lint i = 2; i <= n; ++i) {
    fact[i] = i * fact[i - 1] % m;
  }
}

lint modpow(lint x, lint n, lint m = Mod) {
  lint res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * x % m;
    x = x * x % m;
    n >>= 1;
  }
  return res;
}

lint intpow(lint x, lint n) {
  lint res = 1;
  while (n > 0) {
    if (n & 1)
      res *= x;
    x *= x;
    n >>= 1;
  }
  return res;
}

lint comb(lint n, lint r, lint m = Mod) {
  if (r == 0 || r == n)
    return 1;
  lint res = fact[n] * modpow(fact[n - r], m - 2, m) % m *
             modpow(fact[r], m - 2, m) % m;
  return res < 0 ? res + m : res;
}

map<lint, lint> factring(lint n) {
  map<lint, lint> res;
  for (lint i = 2; i * i <= n; ++i) {
    while (n % i == 0) {
      n /= i;
      ++res[i];
    }
  }
  if (n != 1)
    ++res[n];
  return res;
}

int n, r, w, ans;
string s;
int main() {
  cin >> n >> s;
  rep(i, n) {
    char c = s[i];
    if (c == 'W')
      w++;
    else
      r++;
  }
  rep(i, r) {
    if (s[i] == 'W')
      ans++;
  }
  cout << ans << endl;
}

Error: 0

Fixed submission:

#include <algorithm>
#include <assert.h>
#include <bitset>
#include <cmath>
#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>

using namespace std;
using lint = int64_t;

#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep1(i, n) for (lint i = 1; i < (n); ++i)
#define repn(i, a, b) for (lint i = (a); i < (b); ++i)
#define rep_inv(i, n) for (lint i = (n); i >= 0; --i)
#define all(vec) vec.begin(), vec.end()
#define cend printf("\\n")

// constexpr lint mod = 998'244'353LL;
constexpr lint Mod = 1000'000'007LL;
constexpr lint Inf = 3'000'000'000'000'000'007LL; // 3e18+7
constexpr double Pi = 3.141592653589793;

template <class T> using prique = priority_queue<T>;
template <class T> using prique_inv = priority_queue<T, vector<T>, greater<T>>;
template <class T, class U>
inline istream &operator>>(istream &is, pair<T, U> &rhs) {
  return is >> rhs.first >> rhs.second;
}
template <class T, class U>
inline ostream &operator<<(ostream &os, const pair<T, U> &rhs) {
  return os << rhs.first << " " << rhs.second;
}
template <class InputIterator>
void arrin(InputIterator first, InputIterator last) {
  for (; first != last; ++first)
    cin >> (*first);
}
template <class InputIterator>
void arrout(InputIterator first, InputIterator last) {
  for (; first != last; ++first) {
    cout << (*first) << ((first + 1) != last ? " " : "\\n");
  }
}

bool pri(lint x) {
  for (lint i = 2; i * i <= x; ++i) {
    if (x % i == 0)
      return false;
  }
  return 1 < x;
}

lint fact[3000000];
void fact_init(lint n, lint m = Mod) {
  if (3000000 <= n)
    return;
  fact[0] = fact[1] = 1;
  for (lint i = 2; i <= n; ++i) {
    fact[i] = i * fact[i - 1] % m;
  }
}

lint modpow(lint x, lint n, lint m = Mod) {
  lint res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * x % m;
    x = x * x % m;
    n >>= 1;
  }
  return res;
}

lint intpow(lint x, lint n) {
  lint res = 1;
  while (n > 0) {
    if (n & 1)
      res *= x;
    x *= x;
    n >>= 1;
  }
  return res;
}

lint comb(lint n, lint r, lint m = Mod) {
  if (r == 0 || r == n)
    return 1;
  lint res = fact[n] * modpow(fact[n - r], m - 2, m) % m *
             modpow(fact[r], m - 2, m) % m;
  return res < 0 ? res + m : res;
}

map<lint, lint> factring(lint n) {
  map<lint, lint> res;
  for (lint i = 2; i * i <= n; ++i) {
    while (n % i == 0) {
      n /= i;
      ++res[i];
    }
  }
  if (n != 1)
    ++res[n];
  return res;
}

int n, r, w, ans;
string s;
int main() {
  cin >> n;
  rep(i, n) {
    char c;
    cin >> c;
    if (c == 'W')
      w++;
    else
      r++;

    s[i] = c;
  }
  rep(i, r) {
    if (s[i] == 'W')
      ans++;
  }
  cout << ans << endl;
}