Problem p03057 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
struct fast_ios {
  fast_ios() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
#define FOR(i, begin, end) for (int i = (begin); i < (end); i++)
#define REP(i, n) FOR(i, 0, n)
#define IFOR(i, begin, end) for (int i = (end)-1; i >= (begin); i--)
#define IREP(i, n) IFOR(i, 0, n)
#define Sort(v) sort(v.begin(), v.end())
#define Reverse(v) reverse(v.begin(), v.end())
#define all(v) v.begin(), v.end()
#define SZ(v) ((int)v.size())
#define Lower_bound(v, x)                                                      \
  distance(v.begin(), lower_bound(v.begin(), v.end(), x))
#define Upper_bound(v, x)                                                      \
  distance(v.begin(), upper_bound(v.begin(), v.end(), x))
#define Max(a, b) a = max(a, b)
#define Min(a, b) a = min(a, b)
#define bit(n) (1LL << (n))
#define bit_exist(x, n) ((x >> n) & 1)
#define debug(x) cout << #x << "=" << x << endl;
#define vdebug(v)                                                              \
  cout << #v << "=" << endl;                                                   \
  REP(i_debug, v.size()) { cout << v[i_debug] << ","; }                        \
  cout << endl;
#define mdebug(m)                                                              \
  cout << #m << "=" << endl;                                                   \
  REP(i_debug, m.size()) {                                                     \
    REP(j_debug, m[i_debug].size()) { cout << m[i_debug][j_debug] << ","; }    \
    cout << endl;                                                              \
  }
#define pb push_back
#define f first
#define s second
#define int long long
#define INF 1000000000000000000
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &x : v)
    is >> x;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, vector<T> &v) {
  for (int i = 0; i < v.size(); i++) {
    cout << v[i];
    if (i != v.size() - 1)
      cout << endl;
  };
  return os;
}
template <typename T> void Out(T x) { cout << x << endl; }
template <typename T1, typename T2> void Ans(bool f, T1 y, T2 n) {
  if (f)
    Out(y);
  else
    Out(n);
}

using vec = vector<int>;
using mat = vector<vec>;
using Pii = pair<int, int>;
using PiP = pair<int, Pii>;
using PPi = pair<Pii, int>;
using bools = vector<bool>;
using pairs = vector<Pii>;

// int dx[4] = {1,0,-1,0};
// int dy[4] = {0,1,0,-1};
// char d[4] = {'D','R','U','L'};

const int mod = 1000000007;
// const int mod = 998244353;
#define Add(x, y) x = (x + (y)) % mod
#define Mult(x, y) x = (x * (y)) % mod

template <long long MOD> struct ModInt {

  using ll = long long;
  ll val;

  void setval(ll v) { val = v % MOD; };
  ModInt() : val(0) {}
  ModInt(ll v) { setval(v); };

  ModInt operator+(const ModInt &x) const { return ModInt(val + x.val); }
  ModInt operator-(const ModInt &x) const { return ModInt(val - x.val + MOD); }
  ModInt operator*(const ModInt &x) const { return ModInt(val * x.val); }
  ModInt operator/(const ModInt &x) const { return *this * x.inv(); }
  ModInt operator-() const { return ModInt(MOD - val); }
  ModInt operator+=(const ModInt &x) { return *this = *this + x; }
  ModInt operator-=(const ModInt &x) { return *this = *this - x; }
  ModInt operator*=(const ModInt &x) { return *this = *this * x; }
  ModInt operator/=(const ModInt &x) { return *this = *this / x; }

  friend ostream &operator<<(ostream &os, const ModInt &x) {
    os << x.val;
    return os;
  }
  friend istream &operator>>(istream &is, ModInt &x) {
    is >> x.val;
    x.val = (x.val % MOD + MOD) % MOD;
    return is;
  }

  ModInt pow(ll n) const {
    ModInt a = 1;
    if (n == 0)
      return a;
    int i0 = 64 - __builtin_clzll(n);
    for (int i = i0 - 1; i >= 0; i--) {
      a = a * a;
      if ((n >> i) & 1)
        a *= (*this);
    }
    return a;
  }
  ModInt inv() const { return this->pow(MOD - 2); }
};

using mint = ModInt<mod>;
mint pow(mint x, long long n) { return x.pow(n); }
// using mint = double; //for debug
using mvec = vector<mint>;
using mmat = vector<mvec>;

struct Combination {

  vector<mint> fact, invfact;

  Combination(int N) {
    fact = vector<mint>({mint(1)});
    invfact = vector<mint>({mint(1)});
    fact_initialize(N);
  }

  void fact_initialize(int N) {
    int i0 = fact.size();
    if (i0 >= N + 1)
      return;
    fact.resize(N + 1);
    invfact.resize(N + 1);
    for (int i = i0; i <= N; i++)
      fact[i] = fact[i - 1] * i;
    invfact[N] = (mint)1 / fact[N];
    for (int i = N - 1; i >= i0; i--)
      invfact[i] = invfact[i + 1] * (i + 1);
  }

  mint nCr(int n, int r) {
    if (n < 0 || r < 0 || r > n)
      return mint(0);
    if (fact.size() < n + 1)
      fact_initialize(n);
    return fact[n] * invfact[r] * invfact[n - r];
  }

  mint nPr(int n, int r) {
    if (n < 0 || r < 0 || r > n)
      return mint(0);
    if (fact.size() < n + 1)
      fact_initialize(n);
    return fact[n] * invfact[n - r];
  }
};

struct SegmentTree {
  using T = mint;

  int N;
  vector<T> dat;

  T id = 0;
  T F(T &a, T &b) { return a + b; }

  SegmentTree(int n) {
    N = 1;
    while (n > N)
      N = N << 1;
    dat = vector<T>(2 * N - 1, id);
  }

  SegmentTree(int n, vector<T> &v) {
    N = 1;
    while (n > N)
      N = N << 1;
    dat = vector<T>(2 * N - 1, id);
    for (int i = 0; i < n; i++)
      dat[i + N - 1] = v[i];
    for (int i = N - 2; i >= 0; i--)
      dat[i] = F(dat[i * 2 + 1], dat[i * 2 + 2]);
  }

  void update(int k, T a) {
    k += N - 1;
    dat[k] = a;
    while (k > 0) {
      k = (k - 1) / 2;
      dat[k] = F(dat[k * 2 + 1], dat[k * 2 + 2]);
    }
  }

  void reset() { fill(dat.begin(), dat.end(), id); }

  T get(int a, int b, int k, int l, int r) {
    if (r <= a || b <= l)
      return id;
    if (a <= l && r <= b)
      return dat[k];
    else {
      T vl = get(a, b, k * 2 + 1, l, (l + r) / 2);
      T vr = get(a, b, k * 2 + 2, (l + r) / 2, r);
      return F(vl, vr);
    }
  }
  T get(int a, int b) { return get(a, b, 0, 0, N); }

  T val(int k) { return dat[k + N - 1]; }
};

signed main() {

  int N, M;
  cin >> N >> M;
  string S;
  cin >> S;

  if (S[0] == 'B') {
    REP(i, N) {
      if (S[i] == 'B')
        S[i] = 'R';
      else
        S[i] = 'B';
    }
  }

  int f = -1, s = INF, t = 0;
  REP(i, M) {
    if (S[i] == 'R')
      t++;
    else {
      if (f == -1)
        f = t;
      else if (t % 2)
        Min(s, t);
      t = 0;
    }
  }

  mint ans = 0;
  if (f == -1) {
    mmat dp(N, mvec(2, 0));
    dp[0][0] = 1;
    REP(i, N - 1) {
      dp[i + 1][0] += dp[i][0] + dp[i][1];
      dp[i + 1][1] += dp[i][0];
    }
    ans = dp[N - 1][0] + dp[N - 1][1] + dp[N - 2][0];
  } else if (N % 2 == 0) {
    int d;
    if (f % 2)
      d = min(f, s);
    else
      d = min(f + 1, s);
    // debug(d);
    N /= 2;
    d /= 2;

    mvec dp(N, 0);
    dp[0] = 1;
    SegmentTree ST(N, dp);
    FOR(i, 1, N) {
      dp[i] = ST.get(i - d - 1, i);
      ST.update(i, dp[i]);
    }
    REP(i, d + 1) if (N - 1 - i >= 0) ans += dp[N - 1 - i] * 2 * (i + 1);
    // vdebug(dp);
  }
  Out(ans);

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
struct fast_ios {
  fast_ios() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
#define FOR(i, begin, end) for (int i = (begin); i < (end); i++)
#define REP(i, n) FOR(i, 0, n)
#define IFOR(i, begin, end) for (int i = (end)-1; i >= (begin); i--)
#define IREP(i, n) IFOR(i, 0, n)
#define Sort(v) sort(v.begin(), v.end())
#define Reverse(v) reverse(v.begin(), v.end())
#define all(v) v.begin(), v.end()
#define SZ(v) ((int)v.size())
#define Lower_bound(v, x)                                                      \
  distance(v.begin(), lower_bound(v.begin(), v.end(), x))
#define Upper_bound(v, x)                                                      \
  distance(v.begin(), upper_bound(v.begin(), v.end(), x))
#define Max(a, b) a = max(a, b)
#define Min(a, b) a = min(a, b)
#define bit(n) (1LL << (n))
#define bit_exist(x, n) ((x >> n) & 1)
#define debug(x) cout << #x << "=" << x << endl;
#define vdebug(v)                                                              \
  cout << #v << "=" << endl;                                                   \
  REP(i_debug, v.size()) { cout << v[i_debug] << ","; }                        \
  cout << endl;
#define mdebug(m)                                                              \
  cout << #m << "=" << endl;                                                   \
  REP(i_debug, m.size()) {                                                     \
    REP(j_debug, m[i_debug].size()) { cout << m[i_debug][j_debug] << ","; }    \
    cout << endl;                                                              \
  }
#define pb push_back
#define f first
#define s second
#define int long long
#define INF 1000000000000000000
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &x : v)
    is >> x;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, vector<T> &v) {
  for (int i = 0; i < v.size(); i++) {
    cout << v[i];
    if (i != v.size() - 1)
      cout << endl;
  };
  return os;
}
template <typename T> void Out(T x) { cout << x << endl; }
template <typename T1, typename T2> void Ans(bool f, T1 y, T2 n) {
  if (f)
    Out(y);
  else
    Out(n);
}

using vec = vector<int>;
using mat = vector<vec>;
using Pii = pair<int, int>;
using PiP = pair<int, Pii>;
using PPi = pair<Pii, int>;
using bools = vector<bool>;
using pairs = vector<Pii>;

// int dx[4] = {1,0,-1,0};
// int dy[4] = {0,1,0,-1};
// char d[4] = {'D','R','U','L'};

const int mod = 1000000007;
// const int mod = 998244353;
#define Add(x, y) x = (x + (y)) % mod
#define Mult(x, y) x = (x * (y)) % mod

template <long long MOD> struct ModInt {

  using ll = long long;
  ll val;

  void setval(ll v) { val = v % MOD; };
  ModInt() : val(0) {}
  ModInt(ll v) { setval(v); };

  ModInt operator+(const ModInt &x) const { return ModInt(val + x.val); }
  ModInt operator-(const ModInt &x) const { return ModInt(val - x.val + MOD); }
  ModInt operator*(const ModInt &x) const { return ModInt(val * x.val); }
  ModInt operator/(const ModInt &x) const { return *this * x.inv(); }
  ModInt operator-() const { return ModInt(MOD - val); }
  ModInt operator+=(const ModInt &x) { return *this = *this + x; }
  ModInt operator-=(const ModInt &x) { return *this = *this - x; }
  ModInt operator*=(const ModInt &x) { return *this = *this * x; }
  ModInt operator/=(const ModInt &x) { return *this = *this / x; }

  friend ostream &operator<<(ostream &os, const ModInt &x) {
    os << x.val;
    return os;
  }
  friend istream &operator>>(istream &is, ModInt &x) {
    is >> x.val;
    x.val = (x.val % MOD + MOD) % MOD;
    return is;
  }

  ModInt pow(ll n) const {
    ModInt a = 1;
    if (n == 0)
      return a;
    int i0 = 64 - __builtin_clzll(n);
    for (int i = i0 - 1; i >= 0; i--) {
      a = a * a;
      if ((n >> i) & 1)
        a *= (*this);
    }
    return a;
  }
  ModInt inv() const { return this->pow(MOD - 2); }
};

using mint = ModInt<mod>;
mint pow(mint x, long long n) { return x.pow(n); }
// using mint = double; //for debug
using mvec = vector<mint>;
using mmat = vector<mvec>;

struct Combination {

  vector<mint> fact, invfact;

  Combination(int N) {
    fact = vector<mint>({mint(1)});
    invfact = vector<mint>({mint(1)});
    fact_initialize(N);
  }

  void fact_initialize(int N) {
    int i0 = fact.size();
    if (i0 >= N + 1)
      return;
    fact.resize(N + 1);
    invfact.resize(N + 1);
    for (int i = i0; i <= N; i++)
      fact[i] = fact[i - 1] * i;
    invfact[N] = (mint)1 / fact[N];
    for (int i = N - 1; i >= i0; i--)
      invfact[i] = invfact[i + 1] * (i + 1);
  }

  mint nCr(int n, int r) {
    if (n < 0 || r < 0 || r > n)
      return mint(0);
    if (fact.size() < n + 1)
      fact_initialize(n);
    return fact[n] * invfact[r] * invfact[n - r];
  }

  mint nPr(int n, int r) {
    if (n < 0 || r < 0 || r > n)
      return mint(0);
    if (fact.size() < n + 1)
      fact_initialize(n);
    return fact[n] * invfact[n - r];
  }
};

struct SegmentTree {
  using T = mint;

  int N;
  vector<T> dat;

  T id = 0;
  T F(T &a, T &b) { return a + b; }

  SegmentTree(int n) {
    N = 1;
    while (n > N)
      N = N << 1;
    dat = vector<T>(2 * N - 1, id);
  }

  SegmentTree(int n, vector<T> &v) {
    N = 1;
    while (n > N)
      N = N << 1;
    dat = vector<T>(2 * N - 1, id);
    for (int i = 0; i < n; i++)
      dat[i + N - 1] = v[i];
    for (int i = N - 2; i >= 0; i--)
      dat[i] = F(dat[i * 2 + 1], dat[i * 2 + 2]);
  }

  void update(int k, T a) {
    k += N - 1;
    dat[k] = a;
    while (k > 0) {
      k = (k - 1) / 2;
      dat[k] = F(dat[k * 2 + 1], dat[k * 2 + 2]);
    }
  }

  void reset() { fill(dat.begin(), dat.end(), id); }

  T get(int a, int b, int k, int l, int r) {
    if (r <= a || b <= l)
      return id;
    if (a <= l && r <= b)
      return dat[k];
    else {
      T vl = get(a, b, k * 2 + 1, l, (l + r) / 2);
      T vr = get(a, b, k * 2 + 2, (l + r) / 2, r);
      return F(vl, vr);
    }
  }
  T get(int a, int b) { return get(a, b, 0, 0, N); }

  T val(int k) { return dat[k + N - 1]; }
};

signed main() {

  int N, M;
  cin >> N >> M;
  string S;
  cin >> S;

  if (S[0] == 'B') {
    REP(i, M) {
      if (S[i] == 'B')
        S[i] = 'R';
      else
        S[i] = 'B';
    }
  }

  int f = -1, s = INF, t = 0;
  REP(i, M) {
    if (S[i] == 'R')
      t++;
    else {
      if (f == -1)
        f = t;
      else if (t % 2)
        Min(s, t);
      t = 0;
    }
  }

  mint ans = 0;
  if (f == -1) {
    mmat dp(N, mvec(2, 0));
    dp[0][0] = 1;
    REP(i, N - 1) {
      dp[i + 1][0] += dp[i][0] + dp[i][1];
      dp[i + 1][1] += dp[i][0];
    }
    ans = dp[N - 1][0] + dp[N - 1][1] + dp[N - 2][0];
  } else if (N % 2 == 0) {
    int d;
    if (f % 2)
      d = min(f, s);
    else
      d = min(f + 1, s);
    // debug(d);
    N /= 2;
    d /= 2;

    mvec dp(N, 0);
    dp[0] = 1;
    SegmentTree ST(N, dp);
    FOR(i, 1, N) {
      dp[i] = ST.get(i - d - 1, i);
      ST.update(i, dp[i]);
    }
    REP(i, d + 1) if (N - 1 - i >= 0) ans += dp[N - 1 - i] * 2 * (i + 1);
    // vdebug(dp);
  }
  Out(ans);

  return 0;
}