Problem p02597 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <cassert>
#include <cmath>
#include <deque>
#include <functional>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <vector>
using namespace std;
typedef long long ll;
int dx[4] = {0, 0, 1, -1};
int dy[4] = {1, -1, 0, 0};
#define LNF __LONG_LONG_MAX__
const ll MOD = 1000000007;
const int MAX = 200000;

long long fac[MAX], finv[MAX], inv[MAX];

// テーブルを作る前処理
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// 二項係数計算
long long COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
// 法mでのaの逆元を計算
long long modinv(long long a, long long m) {
  long long b = m, u = 1, v = 0;
  while (b) {
    long long t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= m;
  if (u < 0)
    u += m;
  return u;
}
// a^n mod を計算する
long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}
// 最大公約数
long long gcd(long long x, long long y) {
  if (y == 0)
    return x;
  else
    return gcd(y, x % y);
}
// 最小公倍数
long long lcm(long long x, long long y) { return x / gcd(x, y) * y; }
// nが大きく,kが10^7程度の時の二項係数
long long com[200005];
void combination(long long n, long long k) {
  com[0] = 1;
  for (int i = 1; i <= k; i++) {
    com[i] = (com[i - 1] * (n - i + 1) % MOD) * modinv(i, MOD) % MOD;
  }
}

int main() {
  int N;
  cin >> N;
  string c;
  cin >> c;
  vector<int> r, w;
  for (int i = 0; i < N; i++) {
    if (c[i] == 'R')
      r.push_back(i);
    else
      w.push_back(i);
  }
  if (w.size() == 0) {
    cout << 0 << endl;
    return 0;
  }
  int out = w.size();
  int cnt = 0;
  int ch = 0;
  int cm = r.size() - 1;
  int m = r[cm];
  int h = w[ch];
  while (h < m) {
    ch++;
    cm--;
    h = w[ch];
    m = r[cm];
    cnt++;
  }
  cout << min(out, cnt) << endl;
  return 0;
}


Fixed submission:
#include <algorithm>
#include <cassert>
#include <cmath>
#include <deque>
#include <functional>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <vector>
using namespace std;
typedef long long ll;
int dx[4] = {0, 0, 1, -1};
int dy[4] = {1, -1, 0, 0};
#define LNF __LONG_LONG_MAX__
const ll MOD = 1000000007;
const int MAX = 200000;

long long fac[MAX], finv[MAX], inv[MAX];

// テーブルを作る前処理
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

// 二項係数計算
long long COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
// 法mでのaの逆元を計算
long long modinv(long long a, long long m) {
  long long b = m, u = 1, v = 0;
  while (b) {
    long long t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= m;
  if (u < 0)
    u += m;
  return u;
}
// a^n mod を計算する
long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}
// 最大公約数
long long gcd(long long x, long long y) {
  if (y == 0)
    return x;
  else
    return gcd(y, x % y);
}
// 最小公倍数
long long lcm(long long x, long long y) { return x / gcd(x, y) * y; }
// nが大きく,kが10^7程度の時の二項係数
long long com[200005];
void combination(long long n, long long k) {
  com[0] = 1;
  for (int i = 1; i <= k; i++) {
    com[i] = (com[i - 1] * (n - i + 1) % MOD) * modinv(i, MOD) % MOD;
  }
}

int main() {
  int N;
  cin >> N;
  string c;
  cin >> c;
  vector<int> r, w;
  for (int i = 0; i < N; i++) {
    if (c[i] == 'R')
      r.push_back(i);
    else
      w.push_back(i);
  }
  if (w.size() == 0 || r.size() == 0) {
    cout << 0 << endl;
    return 0;
  }
  int out = w.size();
  int cnt = 0;
  int ch = 0;
  int cm = r.size() - 1;
  int m = r[cm];
  int h = w[ch];
  while (h < m) {
    ch++;
    cm--;
    h = w[ch];
    m = r[cm];
    cnt++;
  }
  cout << min(out, cnt) << endl;
  return 0;
}