Problem p03057 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <cmath>
#include <complex>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <thread>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;
typedef unsigned long long int ull;
typedef long long int ll;
typedef pair<ll, ll> pll;
typedef pair<int, int> pi;
typedef pair<double, double> pd;
typedef pair<double, ll> pdl;
#define F first
#define S second
const ll E = 1e18 + 7;
const ll MOD = 1000000007;

int main() {
  ll n, m;
  cin >> n >> m;
  string s;
  cin >> s;
  if (s[0] == 'B') {
    for (int i = 0; i < m; i++) {
      if (s[i] == 'B') {
        s[i] = 'R';
      } else {
        s[i] = 'B';
      }
    }
  }
  ll l = 0;
  while (l < m && s[l] == 'R') {
    l++;
  }
  if (l == m) {
    assert(false);
  }
  if (n & 1) {
    cout << (l == m ? 1 : 0) << endl;
    return 0;
  }
  ll mi = E;
  if (l & 1) {
    mi = l;
  } else {
    mi = l + 1;
  }
  for (ll i = l; i < m; i++) {
    if (s[i] == 'R') {
      ll w = i;
      while (i < m && s[i] == 'R') {
        i++;
      }
      if (i != m) {
        ll L = i - w;
        if (L & 1) {
          mi = min(mi, L);
        }
      }
    }
  }
  vector<ll> dp(n, 0);
  vector<ll> sum(n, 0);
  dp[0] = sum[0] = 1;
  for (int i = 2; i < n; i += 2) {
    ll r = sum[i - 2];
    ll l = (i - mi - 3 >= 0 ? sum[i - mi - 3] : 0LL);
    dp[i] = r - l;
    dp[i] %= MOD;
    sum[i] = sum[i - 2] + dp[i];
    sum[i] %= MOD;
    // cout<<dp[i]<<" "<<sum[i]<<endl;
  }
  ll ans = 0;
  for (ll i = 2; i <= mi + 1 && n - i >= 0; i++) {
    ans += dp[n - i] * i % MOD;
    ans %= MOD;
  }
  if (l == m) {
    ans++;
  }
  ans %= MOD;
  if (ans < 0) {
    ans += MOD;
  }
  cout << ans << endl;

  return 0;
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <cmath>
#include <complex>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <thread>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;
typedef unsigned long long int ull;
typedef long long int ll;
typedef pair<ll, ll> pll;
typedef pair<int, int> pi;
typedef pair<double, double> pd;
typedef pair<double, ll> pdl;
#define F first
#define S second
const ll E = 1e18 + 7;
const ll MOD = 1000000007;

int main() {
  ll n, m;
  cin >> n >> m;
  string s;
  cin >> s;
  if (s[0] == 'B') {
    for (int i = 0; i < m; i++) {
      if (s[i] == 'B') {
        s[i] = 'R';
      } else {
        s[i] = 'B';
      }
    }
  }
  ll l = 0;
  while (l < m && s[l] == 'R') {
    l++;
  }
  if (l == m) {
    ll ans = 1;
    vector<ll> dp(n, 0);
    vector<ll> sum(n, 0);
    dp[0] = sum[0] = 1;
    for (int i = 2; i < n; i++) {
      dp[i] = sum[i - 2];
      sum[i] = dp[i] + sum[i - 1];
      sum[i] %= MOD;
    }
    for (ll i = 2; i <= n; i++) {
      ans += sum[n - i] * i % MOD;
    }
    ans %= MOD;
    cout << ans << endl;
    return 0;
  }
  if (n & 1) {
    cout << (l == m ? 1 : 0) << endl;
    return 0;
  }
  ll mi = E;
  if (l & 1) {
    mi = l;
  } else {
    mi = l + 1;
  }
  for (ll i = l; i < m; i++) {
    if (s[i] == 'R') {
      ll w = i;
      while (i < m && s[i] == 'R') {
        i++;
      }
      if (i != m) {
        ll L = i - w;
        if (L & 1) {
          mi = min(mi, L);
        }
      }
    }
  }
  vector<ll> dp(n, 0);
  vector<ll> sum(n, 0);
  dp[0] = sum[0] = 1;
  for (int i = 2; i < n; i += 2) {
    ll r = sum[i - 2];
    ll l = (i - mi - 3 >= 0 ? sum[i - mi - 3] : 0LL);
    dp[i] = r - l;
    dp[i] %= MOD;
    sum[i] = sum[i - 2] + dp[i];
    sum[i] %= MOD;
    // cout<<dp[i]<<" "<<sum[i]<<endl;
  }
  ll ans = 0;
  for (ll i = 2; i <= mi + 1 && n - i >= 0; i++) {
    ans += dp[n - i] * i % MOD;
    ans %= MOD;
  }
  if (l == m) {
    ans++;
  }
  ans %= MOD;
  if (ans < 0) {
    ans += MOD;
  }
  cout << ans << endl;

  return 0;
}