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

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

#define PI 3.141592653589
#define ll long long int
#define ld long double
#define vi vector<int>
#define ii pair<int, int>
#define pb push_back
#define mp make_pair
#define ff first
#define ss second
#define pll pair<ll, ll>
#define vv vector
int MOD = 1e9 + 7;
ll power(ll a, ll b) { // a^b
  ll res = 1;
  a = a % MOD;
  while (b > 0) {
    if (b & 1) {
      res = (res * a) % MOD;
      b--;
    }
    a = (a * a) % MOD;
    b >>= 1;
  }
  return res;
}
ll fermat_inv(ll y) { return power(y, MOD - 2); }
ll gcd(ll a, ll b) { return (b == 0) ? a : gcd(b, a % b); }
ll min(ll a, ll b) { return (a > b) ? b : a; }
ll max(ll a, ll b) { return (a > b) ? a : b; }
bool prime[1000001];
vi primes;
void SieveOfEratosthenes(int n) {
  memset(prime, true, sizeof(prime));
  for (int p = 2; p * p <= n; p++) {
    if (prime[p] == true) {
      for (int i = p * p; i <= n; i += p)
        prime[i] = false;
    }
  }
  for (int p = 2; p < 1000001; p++)
    if (prime[p])
      primes.pb(p);
}
ll fact[1001];
void factorial(int n) {
  fact[0] = 1;
  for (int i = 1; i <= n; i++)
    fact[i] = fact[i - 1] * i, fact[i] %= MOD;
}
ll ncr(ll n, ll r) {
  if (n < r)
    return -1;
  else {
    ll x = fact[r] * fact[n - r] % MOD;
    return fact[n] * fermat_inv(x) % MOD;
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif

  int t = 1;
  // cin>>t;
  // SieveOfEratosthenes(1000000);
  // factorial(1000);
  while (t--) {
    int n;
    string s;
    cin >> n >> s;
    int l = n, ans = 0;
    for (int i = n - 1; i >= 0; i--) {
      if (s[i] == 'W')
        l = i;
    }
    for (int i = n - 1; i > l; i--) {
      if (s[i] == 'R') {
        ans++;
        while (l < n && s[++l] != 'W')
          ;
      }
    }
    cout << ans;
  }
}

Error: WA

Fixed submission:

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

#define PI 3.141592653589
#define ll long long int
#define ld long double
#define vi vector<int>
#define ii pair<int, int>
#define pb push_back
#define mp make_pair
#define ff first
#define ss second
#define pll pair<ll, ll>
#define vv vector
int MOD = 1e9 + 7;
ll power(ll a, ll b) { // a^b
  ll res = 1;
  a = a % MOD;
  while (b > 0) {
    if (b & 1) {
      res = (res * a) % MOD;
      b--;
    }
    a = (a * a) % MOD;
    b >>= 1;
  }
  return res;
}
ll fermat_inv(ll y) { return power(y, MOD - 2); }
ll gcd(ll a, ll b) { return (b == 0) ? a : gcd(b, a % b); }
ll min(ll a, ll b) { return (a > b) ? b : a; }
ll max(ll a, ll b) { return (a > b) ? a : b; }
bool prime[1000001];
vi primes;
void SieveOfEratosthenes(int n) {
  memset(prime, true, sizeof(prime));
  for (int p = 2; p * p <= n; p++) {
    if (prime[p] == true) {
      for (int i = p * p; i <= n; i += p)
        prime[i] = false;
    }
  }
  for (int p = 2; p < 1000001; p++)
    if (prime[p])
      primes.pb(p);
}
ll fact[1001];
void factorial(int n) {
  fact[0] = 1;
  for (int i = 1; i <= n; i++)
    fact[i] = fact[i - 1] * i, fact[i] %= MOD;
}
ll ncr(ll n, ll r) {
  if (n < r)
    return -1;
  else {
    ll x = fact[r] * fact[n - r] % MOD;
    return fact[n] * fermat_inv(x) % MOD;
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif

  int t = 1;
  // cin>>t;
  // SieveOfEratosthenes(1000000);
  // factorial(1000);
  while (t--) {
    int n;
    string s;
    cin >> n >> s;
    int l = n, ans = 0;
    for (int i = n - 1; i >= 0; i--) {
      if (s[i] == 'W')
        l = i;
    }
    for (int i = n - 1; i > l; i--) {
      if (s[i] == 'R') {
        ans++;
        while (s[++l] != 'W')
          ;
      }
    }
    cout << ans;
  }
}