Problem p02929 (C++)

Buggy submission (status: Runtime Error):
/*
;<+111===1?=1=zzzzvI==llllltttttttrrrrrrrvvwXzzzzzzzzzzzzzzzzzzzzzzzzzzzuUXwzzXXzvvvvvvvvXXwrrrrrrrrttz-_...............
:;;;;;;;;;;;<<?<+??=====lllzOtOZOtttttrrrrw0vvzzzzzzzuuuuuuuuuuuuuuuuuuuuuuXWXXXUXzzzzwwvvvwUwvvrrrrrrttt+_.............
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;>;>>+z<(+><<+<<?<(zI?==z0OZlllllltttrwIzrrXv+OWuuuuuuuZZZZZyWWkZyZZZZyZZZZZZZZuuuuXk?wkzzzvvvvrvZXrrrrZzttto~~~..~.....
>>>>jI+wI+=1z<+==1zdZllzZOZttltltttrwVOwvzzuWXzvWuZZZZZyyyWfWWyyWWWkVVyWWVyyyyZZZZZX0zXkuuzzvvvvrrrrrrtrzdO1z>~~~~.~.~..
>??zwZwOtOOtlttOOtOXWyrdkdrrrrrrvwwVOwuXWSuZWXWyvWXyyyyyyyVVVVWHWppWWkkWkWHWVyyyyyyXkXWWkXuuuzzvvvvrrrzzOzXy?O-~~~.~~.~~
??jZzzXzw0vrvwXvvwzXqHkWHkuuzuXXZXwXXWgHWZZyWyyWWAXWWVffffWffppHWppppppppppHkpffffWWWHyyHHXZZZuXXuuuuuzOOwwZkzI~~~~~~.~.
??1?=XXXSuXXZuuuuZZXHg@WHHyXkpWWkHH@HHHVyVVVWHffffWWWHWpppWbppbHMHbppppHWpbpbHWbpWHHpHWpW@NkWHWykXWXyZyZkwwXyXWc~~~~~~~~
???=zW0WWWyyyyyyyyVVW@@@@MHpWWH@M@HHppHppfpppHpppHpppWHmHbWHbbkHH@HkbbkbHHkkkkHHkkkkkHNkHHHNWHHpppkWHWpppppkXpffk+-~~~~~
1zuwXWyVVVVWWWWHpffppWMHH@HHHbbbWH@NkpHHppppWUHbbHkbbbkbbHHHkkmkHUMMHkkkqMMHqkkkHHqqqMMqM###MHMqkkkkHWHHHkqqkqqkkWpk&J_~
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=====llWfpSdHbHHMHkkkkqMHHH#MHHHHMMZXHkuV4bbHXZ?HbHqkkbkkkkHHkHWkH_?4UWHqkHHXZWMHmMMMNNMNNNNNNMgg@H@@@@MNMMMMMM@MMgqR:::
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~._<jMMMMNNMNmm2<1w#MMNMM@@@MM@HMNMNNMNMNNN$7MP;;=::
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rtttttttltllllllltlttwgHMNNNN#HMx.....`.````.````````````.._```````````.``.`.``.``.`....(M#NNMMM@@M@Mz?>>;;;;:::::::::::
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vrvrrrrrrrrrrrttttrrrrrdgg@MNMMNN#Rrtllll=======WHa-.-....`..`.....-((+WHHHWyy#Il====lldM@@H@@H@@@HgR=??>>>>>>>>>;;;;;;;
vrvvrrrrrtttrtrrtrtttrrdgggMN@MNN#Krtllllllll=lldkuuXUUG&---.-((JzuuuZZZyyyyyXRlllllllld@@@@@H@@@@MgS==???>>>>>>>;>>>;>;
vrvrrrrrrrrrttrttrrrrrrdHggM#@MNN##rtttllllllllltWuuZuuuuuuuuZuZZZZZZyyyyyyyyXSllllllltWg@@@HW@@@MMg0==?????>>>>>>>>>>>>
rrrrrrrrtrrtrttrtttrtrrdHqggMHHNN#NrtttttlltlllltdkuZZZZZZZZZZZZZZyZyyyyyyyyyWRtllllltdgg@@HHW@@@MgHI==????>??>>>>>>>>>>
rrrrrrrrttttttttttttttrwHkqqHMXM##MwrttttttttttttwKuuZZZZZZZZZZZZZZZZZZZyyyyyyHAwtttttdgggmHXHqmMHqKl=?????>??>??>??>???
vvvrrrtrtttttrttttttttrwHkkkkkkWHHHkrrtttOwwwwXUuXWuuuZuZZZZZZZZZZZZZZZZZZZZyyyyyyWWyqHHHkHZdXWHMkHS===??????????>??????
rrrrrtttttrttttttttttttrWpppppRZMHHkwXUUXvvvzzzzuuuuuuuuuuuuZuuZZZZZZZZZZZZZZZZZZZZZXHHHkH0wXSXHppW7?<11zz???>>?>??>?>??
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rrrtrttttv<!````````.?...UZyXZVWwM@b`
?1OZXUUUUUUUZZXkXuuZuuuuuuZuZXXXkkXXyyWWWWWUWH9=(HHXkHXHbpff~~~.~~~~(-````.``._?<z
rrrtttz^_`````.``.``.!...jZykXdfWX@H-...._?zrrvvzzuuuuuUUuuuuuuZXUuuuuuuuuuuuuuzVOV:.(WHWqqWqkkpW>~~~~~~~~~(-.``.```.`..
*/

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

#define int long long
#define pb push_back
#define F first
#define S second
#define FOR(i, a, b) for (int(i) = (a); (i) < (b); (i)++)
#define REP(i, n) FOR(i, 0, n)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define SORT(c) sort((c).begin(), (c).end())
#define ve vector
#define vi vector<int>
#define vp vector<pair<int, int>>
#define vvi vector<vector<int>>

template <typename T> using pq = priority_queue<T, vector<T>, greater<T>>;
using ll = long long;
ll INF = LLONG_MAX / 4 - 100;
ll mod = 1e9 + 7;
int dx[] = {-1, 0, 1, 0}, dy[] = {0, 1, 0, -1};
vector<ll> prime;

class fact {
public:
  int fmod = 1e9 + 7;
  vector<int> fac, inv;
  fact(int n, int Mod = 1e9 + 7) {
    fmod = Mod;
    fac = vector<int>(n, 0);
    inv = vector<int>(n, 0);
    fac[0] = 1;
    for (int i = 1; i < n; i++)
      fac[i] = fac[i - 1] * i % fmod;
    for (int i = 0; i < n; i++)
      inv[i] = fact::POW(fac[i], fmod - 2);
  }
  ll nCr(ll n, ll r) { return (fac[n] * inv[r] % fmod) * inv[n - r] % fmod; }
  ll POW(ll a, ll b) {
    ll c = 1;
    while (b > 0) {
      if (b & 1) {
        c = a * c % fmod;
      }
      a = a * a % fmod;
      b >>= 1;
    }
    return c;
  }
  inline int operator[](int i) { return fac[i]; }
};

template <class T = ll> T in() {
  T x;
  cin >> x;
  return (x);
}
void DEBUG(vector<int> a) {
  for (int i = 0; i < a.size(); i++)
    cout << a[i] << " ";
  cout << endl;
}
void EMP(int x) { cout << "!!!" << x << "!!!" << endl; }
ll GCD(ll a, ll b) {
  ll c;
  while (b != 0) {
    c = a % b;
    a = b;
    b = c;
  }
  return a;
}
ll LCM(ll a, ll b) { return (a / GCD(a, b)) * (b / GCD(a, b)) * GCD(a, b); }
ll POW(ll a, ll b) {
  ll c = 1;
  while (b > 0) {
    if (b & 1) {
      c = a * c % mod;
    }
    a = a * a % mod;
    b >>= 1;
  }
  return c;
}
void PRI(ll n) {
  bool a[n + 1LL];
  for (int i = 0; i < n + 1LL; i++) {
    a[i] = 1LL;
  }
  for (int i = 2; i < n + 1LL; i++) {
    if (a[i]) {
      prime.pb(i);
      ll b = i;
      while (b <= n) {
        a[b] = 0;
        b += i;
      }
    }
  }
}
template <typename T> T chmin(T &a, T b) {
  if (a > b)
    a = b;
  return a;
}
template <typename T> T chmax(T &a, T b) {
  if (a < b)
    a = b;
  return b;
}
bool isSqrt(ll a) { return pow(sqrt(a), 2) == a ? 1 : 0; }
void YesNo(bool a) {
  if (a)
    cout << "Yes";
  else
    cout << "No";
  cout << endl;
}
void yesno(bool a) {
  if (a)
    cout << "yes";
  else
    cout << "no";
  cout << endl;
}
void YESNO(bool a) {
  if (a)
    cout << "YES";
  else
    cout << "NO";
  cout << endl;
}
double dis(int x1, int x2, int y1, int y2) {
  return sqrt((double)abs(x1 - x2) * (double)abs(x1 - x2) +
              (double)abs(y1 - y2) * (double)abs(y1 - y2));
}

class UnionFind {
private:
  vector<int> par;

public:
  UnionFind(int N) { par = vector<int>(N, -1); }
  int find(int x);
  ll size(int x);
  void unite(int x, int y);
  bool same(int x, int y);
};

int UnionFind::find(int x) {
  if (par[x] < 0)
    return x;
  else
    return par[x] = find(par[x]);
}

ll UnionFind::size(int x) { return -par[find(x)]; }

void UnionFind::unite(int x, int y) {
  x = find(x);
  y = find(y);
  if (x == y)
    return;
  // 大きい方に小さい方をくっ付ける
  if (size(x) < size(y))
    swap(x, y);
  par[x] += par[y];
  par[y] = x;
}

bool UnionFind::same(int x, int y) {
  x = find(x);
  y = find(y);
  return x == y;
}

int inout[100005];
int cnt[100005];

bool solve() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  REP(i, n * 2) {
    if (s[i] == 'B') {
      cnt[i] = 1;
    }
    // cout << cnt[i] << " ";
  }
  if (s[0] == 'W') {
    cout << 0 << endl;
  } else {
    int tmp = 0;
    int pos = 1;
    inout[0] = 1;
    tmp = 1;
    REP(i, n * 2) {
      // cout << tmp << endl;
      if (inout[i] == 2)
        continue;
      FOR(j, pos, n * 2) {
        if (tmp % 2 == cnt[j]) {
          inout[j] = 2;
          pos = j + 1;
          tmp--;
          break;
        } else {
          tmp++;
          inout[j] = 1;
        }
      }
    }
    if (tmp == 0) {
      int ans = 1;
      tmp = 0;
      // REP (i,n * 2) cout << inout[i] << " "; cout << endl;
      REP(i, n * 2) {
        if (inout[i] == 2) {
          ans *= tmp;
          ans %= mod;
          tmp--;
        } else {
          tmp++;
        }
      }
      int mul = 1;
      REP(i, n) {
        mul *= (i + 1);
        mul %= mod;
      }
      cout << ans * mul % mod << endl;
    } else {
      cout << 0 << endl;
    }
  }
}

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
}


Fixed submission:
/*
;<+111===1?=1=zzzzvI==llllltttttttrrrrrrrvvwXzzzzzzzzzzzzzzzzzzzzzzzzzzzuUXwzzXXzvvvvvvvvXXwrrrrrrrrttz-_...............
:;;;;;;;;;;;<<?<+??=====lllzOtOZOtttttrrrrw0vvzzzzzzzuuuuuuuuuuuuuuuuuuuuuuXWXXXUXzzzzwwvvvwUwvvrrrrrrttt+_.............
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;;;;;;;+<~(<<~(<(?<+I??==zwOOOllllltttrwOrwkZwXuzuuuuuuuuXWkZuZZuuZZuZZuuuuuuuuuuuXSI1wkzvvvvvvwXwvrrrrrtttO_~..........
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>>>>jI+wI+=1z<+==1zdZllzZOZttltltttrwVOwvzzuWXzvWuZZZZZyyyWfWWyyWWWkVVyWWVyyyyZZZZZX0zXkuuzzvvvvrrrrrrtrzdO1z>~~~~.~.~..
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1zuwXWyVVVVWWWWHpffppWMHH@HHHbbbWH@NkpHHppppWUHbbHkbbbkbbHHHkkmkHUMMHkkkqMMHqkkkHHqqqMMqM###MHMqkkkkHWHHHkqqkqqkkWpk&J_~
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=====llWfpSdHbHHMHkkkkqMHHH#MHHHHMMZXHkuV4bbHXZ?HbHqkkbkkkkHHkHWkH_?4UWHqkHHXZWMHmMMMNNMNNNNNNMgg@H@@@@MNMMMMMM@MMgqR:::
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vrvrrrrrrrrrttrttrrrrrrdHggM#@MNN##rtttllllllllltWuuZuuuuuuuuZuZZZZZZyyyyyyyyXSllllllltWg@@@HW@@@MMg0==?????>>>>>>>>>>>>
rrrrrrrrtrrtrttrtttrtrrdHqggMHHNN#NrtttttlltlllltdkuZZZZZZZZZZZZZZyZyyyyyyyyyWRtllllltdgg@@HHW@@@MgHI==????>??>>>>>>>>>>
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rrrtttz^_`````.``.``.!...jZykXdfWX@H-...._?zrrvvzzuuuuuUUuuuuuuZXUuuuuuuuuuuuuuzVOV:.(WHWqqWqkkpW>~~~~~~~~~(-.``.```.`..
*/

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

#define int long long
#define pb push_back
#define F first
#define S second
#define FOR(i, a, b) for (int(i) = (a); (i) < (b); (i)++)
#define REP(i, n) FOR(i, 0, n)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define SORT(c) sort((c).begin(), (c).end())
#define ve vector
#define vi vector<int>
#define vp vector<pair<int, int>>
#define vvi vector<vector<int>>

template <typename T> using pq = priority_queue<T, vector<T>, greater<T>>;
using ll = long long;
ll INF = LLONG_MAX / 4 - 100;
ll mod = 1e9 + 7;
int dx[] = {-1, 0, 1, 0}, dy[] = {0, 1, 0, -1};
vector<ll> prime;

class fact {
public:
  int fmod = 1e9 + 7;
  vector<int> fac, inv;
  fact(int n, int Mod = 1e9 + 7) {
    fmod = Mod;
    fac = vector<int>(n, 0);
    inv = vector<int>(n, 0);
    fac[0] = 1;
    for (int i = 1; i < n; i++)
      fac[i] = fac[i - 1] * i % fmod;
    for (int i = 0; i < n; i++)
      inv[i] = fact::POW(fac[i], fmod - 2);
  }
  ll nCr(ll n, ll r) { return (fac[n] * inv[r] % fmod) * inv[n - r] % fmod; }
  ll POW(ll a, ll b) {
    ll c = 1;
    while (b > 0) {
      if (b & 1) {
        c = a * c % fmod;
      }
      a = a * a % fmod;
      b >>= 1;
    }
    return c;
  }
  inline int operator[](int i) { return fac[i]; }
};

template <class T = ll> T in() {
  T x;
  cin >> x;
  return (x);
}
void DEBUG(vector<int> a) {
  for (int i = 0; i < a.size(); i++)
    cout << a[i] << " ";
  cout << endl;
}
void EMP(int x) { cout << "!!!" << x << "!!!" << endl; }
ll GCD(ll a, ll b) {
  ll c;
  while (b != 0) {
    c = a % b;
    a = b;
    b = c;
  }
  return a;
}
ll LCM(ll a, ll b) { return (a / GCD(a, b)) * (b / GCD(a, b)) * GCD(a, b); }
ll POW(ll a, ll b) {
  ll c = 1;
  while (b > 0) {
    if (b & 1) {
      c = a * c % mod;
    }
    a = a * a % mod;
    b >>= 1;
  }
  return c;
}
void PRI(ll n) {
  bool a[n + 1LL];
  for (int i = 0; i < n + 1LL; i++) {
    a[i] = 1LL;
  }
  for (int i = 2; i < n + 1LL; i++) {
    if (a[i]) {
      prime.pb(i);
      ll b = i;
      while (b <= n) {
        a[b] = 0;
        b += i;
      }
    }
  }
}
template <typename T> T chmin(T &a, T b) {
  if (a > b)
    a = b;
  return a;
}
template <typename T> T chmax(T &a, T b) {
  if (a < b)
    a = b;
  return b;
}
bool isSqrt(ll a) { return pow(sqrt(a), 2) == a ? 1 : 0; }
void YesNo(bool a) {
  if (a)
    cout << "Yes";
  else
    cout << "No";
  cout << endl;
}
void yesno(bool a) {
  if (a)
    cout << "yes";
  else
    cout << "no";
  cout << endl;
}
void YESNO(bool a) {
  if (a)
    cout << "YES";
  else
    cout << "NO";
  cout << endl;
}
double dis(int x1, int x2, int y1, int y2) {
  return sqrt((double)abs(x1 - x2) * (double)abs(x1 - x2) +
              (double)abs(y1 - y2) * (double)abs(y1 - y2));
}

class UnionFind {
private:
  vector<int> par;

public:
  UnionFind(int N) { par = vector<int>(N, -1); }
  int find(int x);
  ll size(int x);
  void unite(int x, int y);
  bool same(int x, int y);
};

int UnionFind::find(int x) {
  if (par[x] < 0)
    return x;
  else
    return par[x] = find(par[x]);
}

ll UnionFind::size(int x) { return -par[find(x)]; }

void UnionFind::unite(int x, int y) {
  x = find(x);
  y = find(y);
  if (x == y)
    return;
  // 大きい方に小さい方をくっ付ける
  if (size(x) < size(y))
    swap(x, y);
  par[x] += par[y];
  par[y] = x;
}

bool UnionFind::same(int x, int y) {
  x = find(x);
  y = find(y);
  return x == y;
}

int inout[200005];
int cnt[200005];

bool solve() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  REP(i, n * 2) {
    if (s[i] == 'B') {
      cnt[i] = 1;
    }
    // cout << cnt[i] << " ";
  }
  if (s[0] == 'W') {
    cout << 0 << endl;
  } else {
    int tmp = 0;
    int pos = 1;
    inout[0] = 1;
    tmp = 1;
    REP(i, n * 2) {
      // cout << tmp << endl;
      if (inout[i] == 2)
        continue;
      FOR(j, pos, n * 2) {
        if (tmp % 2 == cnt[j]) {
          inout[j] = 2;
          pos = j + 1;
          tmp--;
          break;
        } else {
          tmp++;
          inout[j] = 1;
        }
      }
    }
    if (tmp == 0) {
      int ans = 1;
      tmp = 0;
      // REP (i,n * 2) cout << inout[i] << " "; cout << endl;
      REP(i, n * 2) {
        if (inout[i] == 2) {
          ans *= tmp;
          ans %= mod;
          tmp--;
        } else {
          tmp++;
        }
      }
      int mul = 1;
      REP(i, n) {
        mul *= (i + 1);
        mul %= mod;
      }
      cout << ans * mul % mod << endl;
    } else {
      cout << 0 << endl;
    }
  }
}

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
}