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

#include <algorithm>
#include <bitset>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <math.h>
#include <memory>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdio.h>
#include <stdlib.h>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;

using ll = long long;
using ld = long double;
using Pii = pair<int, int>;
using Pil = pair<int, ll>;
using Pll = pair<ll, ll>;
using Pid = pair<int, ld>;
using Pis = pair<int, string>;
#define rep(i, n) for (int i = 0; i < n; i++)
#define repm(i, s, n) for (int i = s; i < n; i++)

const int INF = 1 << 30;
const ll INF_L = 1LL << 60;
const int MOD = 1e9 + 7; // 998244353;

void cout_ynl(bool b) { cout << (b ? "Yes" : "No") << endl; }
void cout_YNU(bool b) { cout << (b ? "YES" : "NO") << endl; }
void cout_setprecision(ld ld, int n) {
  cout << fixed << setprecision(n) << ld << endl;
}

// ----------------------------------------------------------------
// String Functions
// ----------------------------------------------------------------
int ctoi(char c) {
  if (isdigit(c))
    return c - '0';
  else if (islower(c))
    return c - 'a';
  else if (isupper(c))
    return c - 'A';
  else
    return -1;
}
char itocd(int i) {
  char c = i + '0';
  if (isdigit(c))
    return c;
  else
    return 0x00;
}
char itocl(int i) {
  char c = i + 'a';
  if (islower(c))
    return c;
  else
    return 0x00;
}
char itocu(int i) {
  char c = i + 'A';
  if (isupper(c))
    return c;
  else
    return 0x00;
}
// ----------------------------------------------------------------

// ----------------------------------------------------------------
// Dynamical Programming
// ----------------------------------------------------------------
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;
}
// ----------------------------------------------------------------

// ----------------------------------------------------------------
// Graph Theory
// ----------------------------------------------------------------
vector<vector<int>> adjMat;
vector<set<Pii>> adjList;

void graphDijkstra() {}
void graphBellmanFord() {}
void graphWarshallFloyd() {}
// ----------------------------------------------------------------

// ----------------------------------------------------------------
// Mathematical Functions
// ----------------------------------------------------------------
ll gcd(ll A, ll B) {
  if (A % B == 0) {
    return (B);
  } else {
    return (gcd(B, A % B));
  }
}
ll lcm(ll A, ll B) { return A * B / gcd(A, B); }
ll divTimes(ll N, ll D) {
  ll res = 0;
  while (N % D == 0) {
    N /= D;
    res++;
  }
  return res;
}

ll powMod(ll B, ll P) {
  if (P == 0)
    return 1;
  if (P % 2 == 0) {
    ll t = powMod(B, P / 2);
    return t * t % MOD;
  }
  return B * powMod(B, P - 1) % MOD;
}

/* ----------------------------------
 Factorial, Permutation, Combination
 ---------------------------------- */
const int FAC_INIT_SIZE = 1e6 + 9;
vector<ll> fac, finv, inv;

void factModInit() {
  fac.resize(FAC_INIT_SIZE);
  finv.resize(FAC_INIT_SIZE);
  inv.resize(FAC_INIT_SIZE);
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < FAC_INIT_SIZE; 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;
  }
}
ll factMod(ll N) { return fac[N] % MOD; }
ll factInvMod(ll N) { return finv[N] % MOD; }
ll permMod(ll N, ll K) {
  if (N < 0 || K < 0 || N < K)
    return 0;
  else
    return factMod(N) * factInvMod(N - K) % MOD;
}
ll combMod(ll N, ll K) {
  if (N < 0 || K < 0 || N < K)
    return 0;
  else if (N < FAC_INIT_SIZE) {
    return factMod(N) * (factInvMod(K) * factInvMod(N - K) % MOD) % MOD;
  } else {
    ll ans = 1;
    ll Ks = K < N - K ? K : N - K;
    for (ll i = N; i > N - Ks; i--) {
      ans *= i;
      ans %= MOD;
    }
    return ans * factInvMod(Ks) % MOD;
  }
}

/* ----------------------------------
 Sieve of Atkin & Bernstein
 ---------------------------------- */
vector<bool> sieve;
void sieveInit(ll size) {
  sieve.clear();
  if (size < 0 || size > INF)
    return;

  int ss = (int)size + 9;
  int ssq = sqrt(ss + 0.1);
  sieve.resize(ss);
  sieve[2] = sieve[3] = true;

  int n = 0;
  for (int z = 1; z <= 5; z += 4) {
    for (int y = z; y <= ssq; y += 6) {
      for (int x = 1; x <= ssq && (n = 4 * x * x + y * y) < ss; x++)
        sieve[n] = !sieve[n];
      for (int x = y + 1; x <= ssq && (n = 3 * x * x - y * y) < ss; x += 2)
        sieve[n] = !sieve[n];
    }
  }
  for (int z = 2; z <= 4; z += 2) {
    for (int y = z; y <= ssq; y += 6) {
      for (int x = 1; x <= ssq && (n = 3 * x * x + y * y) < ss; x += 2)
        sieve[n] = !sieve[n];
      for (int x = y + 1; x <= ssq && (n = 3 * x * x - y * y) < ss; x += 2)
        sieve[n] = !sieve[n];
    }
  }
  for (int z = 1; z <= 2; z++) {
    for (int y = 3; y <= ssq; y += 6) {
      for (int x = z; x <= ssq && (n = 4 * x * x + y * y) < ss; x += 3)
        sieve[n] = !sieve[n];
    }
  }
  for (int k = 5; k <= ssq; k++)
    if (sieve[k])
      for (int n = k * k; n < ss; n += k * k)
        sieve[n] = false;
}

/* ----------------------------------
 Prime Factorization
---------------------------------- */
map<ll, set<ll>> primeFactorSet;
map<ll, map<ll, ll>> primeFactorMap;
void primeFactorInit() {
  primeFactorMap.clear();
  primeFactorSet.clear();
}
void primeFactor(ll N) {
  if (N < 0 || N > INF_L)
    return;

  ll Nc = N;
  ll Nsq = sqrt(N + 0.1);
  set<ll> Nset = {1, N};
  map<ll, ll> Nmap;

  for (int k = 2; k <= Nsq; k++) {
    ll dt = 0;
    ll d = 1;
    while (Nc % k == 0) {
      dt++;
      d *= k;
      Nset.insert(d);
      Nc /= k;
    }
    if (dt > 0)
      Nmap[k] = dt;
    if (Nc == 1)
      break;
  }
  if (Nc != 1) {
    Nmap[Nc] = 1;
    Nset.insert(Nc);
  }

  primeFactorSet[N] = Nset;
  primeFactorMap[N] = Nmap;
}
// ----------------------------------------------------------------

void solve() {

  int N;
  cin >> N;
  string S;
  cin >> S;

  vector<int> idxW, idxR;
  rep(i, N) {
    if (S[i] == 'W')
      idxW.push_back(i);
    else
      idxR.push_back(i);
  }
  sort(idxW.begin(), idxW.end());
  sort(idxR.begin(), idxR.end(), greater<int>());
  int sW = (int)(idxW.size()), sR = (int)(idxR.size());

  int res = 0;
  if (!idxW.empty() && !idxR.empty() && idxW[0] < idxR[0]) {
    while (res < min(sW, sR)) {
      if (idxW[res] > idxR[res])
        break;
      res++;
    }
  }
  cout << res << endl;
}

int main() {

  std::ifstream in("input.txt");
  std::cin.rdbuf(in.rdbuf());

  cin.tie(0);
  ios::sync_with_stdio(false);

  solve();
  return 0;
}


Error: WA

Fixed submission:

#include <algorithm>
#include <bitset>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <math.h>
#include <memory>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdio.h>
#include <stdlib.h>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;

using ll = long long;
using ld = long double;
using Pii = pair<int, int>;
using Pil = pair<int, ll>;
using Pll = pair<ll, ll>;
using Pid = pair<int, ld>;
using Pis = pair<int, string>;
#define rep(i, n) for (int i = 0; i < n; i++)
#define repm(i, s, n) for (int i = s; i < n; i++)

const int INF = 1 << 30;
const ll INF_L = 1LL << 60;
const int MOD = 1e9 + 7; // 998244353;

void cout_ynl(bool b) { cout << (b ? "Yes" : "No") << endl; }
void cout_YNU(bool b) { cout << (b ? "YES" : "NO") << endl; }
void cout_setprecision(ld ld, int n) {
  cout << fixed << setprecision(n) << ld << endl;
}

// ----------------------------------------------------------------
// String Functions
// ----------------------------------------------------------------
int ctoi(char c) {
  if (isdigit(c))
    return c - '0';
  else if (islower(c))
    return c - 'a';
  else if (isupper(c))
    return c - 'A';
  else
    return -1;
}
char itocd(int i) {
  char c = i + '0';
  if (isdigit(c))
    return c;
  else
    return 0x00;
}
char itocl(int i) {
  char c = i + 'a';
  if (islower(c))
    return c;
  else
    return 0x00;
}
char itocu(int i) {
  char c = i + 'A';
  if (isupper(c))
    return c;
  else
    return 0x00;
}
// ----------------------------------------------------------------

// ----------------------------------------------------------------
// Dynamical Programming
// ----------------------------------------------------------------
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;
}
// ----------------------------------------------------------------

// ----------------------------------------------------------------
// Graph Theory
// ----------------------------------------------------------------
vector<vector<int>> adjMat;
vector<set<Pii>> adjList;

void graphDijkstra() {}
void graphBellmanFord() {}
void graphWarshallFloyd() {}
// ----------------------------------------------------------------

// ----------------------------------------------------------------
// Mathematical Functions
// ----------------------------------------------------------------
ll gcd(ll A, ll B) {
  if (A % B == 0) {
    return (B);
  } else {
    return (gcd(B, A % B));
  }
}
ll lcm(ll A, ll B) { return A * B / gcd(A, B); }
ll divTimes(ll N, ll D) {
  ll res = 0;
  while (N % D == 0) {
    N /= D;
    res++;
  }
  return res;
}

ll powMod(ll B, ll P) {
  if (P == 0)
    return 1;
  if (P % 2 == 0) {
    ll t = powMod(B, P / 2);
    return t * t % MOD;
  }
  return B * powMod(B, P - 1) % MOD;
}

/* ----------------------------------
 Factorial, Permutation, Combination
 ---------------------------------- */
const int FAC_INIT_SIZE = 1e6 + 9;
vector<ll> fac, finv, inv;

void factModInit() {
  fac.resize(FAC_INIT_SIZE);
  finv.resize(FAC_INIT_SIZE);
  inv.resize(FAC_INIT_SIZE);
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < FAC_INIT_SIZE; 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;
  }
}
ll factMod(ll N) { return fac[N] % MOD; }
ll factInvMod(ll N) { return finv[N] % MOD; }
ll permMod(ll N, ll K) {
  if (N < 0 || K < 0 || N < K)
    return 0;
  else
    return factMod(N) * factInvMod(N - K) % MOD;
}
ll combMod(ll N, ll K) {
  if (N < 0 || K < 0 || N < K)
    return 0;
  else if (N < FAC_INIT_SIZE) {
    return factMod(N) * (factInvMod(K) * factInvMod(N - K) % MOD) % MOD;
  } else {
    ll ans = 1;
    ll Ks = K < N - K ? K : N - K;
    for (ll i = N; i > N - Ks; i--) {
      ans *= i;
      ans %= MOD;
    }
    return ans * factInvMod(Ks) % MOD;
  }
}

/* ----------------------------------
 Sieve of Atkin & Bernstein
 ---------------------------------- */
vector<bool> sieve;
void sieveInit(ll size) {
  sieve.clear();
  if (size < 0 || size > INF)
    return;

  int ss = (int)size + 9;
  int ssq = sqrt(ss + 0.1);
  sieve.resize(ss);
  sieve[2] = sieve[3] = true;

  int n = 0;
  for (int z = 1; z <= 5; z += 4) {
    for (int y = z; y <= ssq; y += 6) {
      for (int x = 1; x <= ssq && (n = 4 * x * x + y * y) < ss; x++)
        sieve[n] = !sieve[n];
      for (int x = y + 1; x <= ssq && (n = 3 * x * x - y * y) < ss; x += 2)
        sieve[n] = !sieve[n];
    }
  }
  for (int z = 2; z <= 4; z += 2) {
    for (int y = z; y <= ssq; y += 6) {
      for (int x = 1; x <= ssq && (n = 3 * x * x + y * y) < ss; x += 2)
        sieve[n] = !sieve[n];
      for (int x = y + 1; x <= ssq && (n = 3 * x * x - y * y) < ss; x += 2)
        sieve[n] = !sieve[n];
    }
  }
  for (int z = 1; z <= 2; z++) {
    for (int y = 3; y <= ssq; y += 6) {
      for (int x = z; x <= ssq && (n = 4 * x * x + y * y) < ss; x += 3)
        sieve[n] = !sieve[n];
    }
  }
  for (int k = 5; k <= ssq; k++)
    if (sieve[k])
      for (int n = k * k; n < ss; n += k * k)
        sieve[n] = false;
}

/* ----------------------------------
 Prime Factorization
---------------------------------- */
map<ll, set<ll>> primeFactorSet;
map<ll, map<ll, ll>> primeFactorMap;
void primeFactorInit() {
  primeFactorMap.clear();
  primeFactorSet.clear();
}
void primeFactor(ll N) {
  if (N < 0 || N > INF_L)
    return;

  ll Nc = N;
  ll Nsq = sqrt(N + 0.1);
  set<ll> Nset = {1, N};
  map<ll, ll> Nmap;

  for (int k = 2; k <= Nsq; k++) {
    ll dt = 0;
    ll d = 1;
    while (Nc % k == 0) {
      dt++;
      d *= k;
      Nset.insert(d);
      Nc /= k;
    }
    if (dt > 0)
      Nmap[k] = dt;
    if (Nc == 1)
      break;
  }
  if (Nc != 1) {
    Nmap[Nc] = 1;
    Nset.insert(Nc);
  }

  primeFactorSet[N] = Nset;
  primeFactorMap[N] = Nmap;
}
// ----------------------------------------------------------------

void solve() {

  int N;
  cin >> N;
  string S;
  cin >> S;

  vector<int> idxW, idxR;
  rep(i, N) {
    if (S[i] == 'W')
      idxW.push_back(i);
    else
      idxR.push_back(i);
  }
  sort(idxW.begin(), idxW.end());
  sort(idxR.begin(), idxR.end(), greater<int>());

  int res = 0;
  if (!idxW.empty() && !idxR.empty()) {
    while (true) {
      res++;
      if (idxW[res] > idxR[res])
        break;
    }
  }
  cout << res << endl;
}

int main() {

  std::ifstream in("input.txt");
  std::cin.rdbuf(in.rdbuf());

  cin.tie(0);
  ios::sync_with_stdio(false);

  solve();
  return 0;
}