Problem p04046 (C++)

Buggy submission (status: Runtime Error):
// #pragma comment(linker, "/stack:200000000")
// #pragma GCC optimize("O3")
// #pragma GCC optimize("Ofast")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
// #pragma GCC optimize("unroll-loops")

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#ifdef on_linux
#include <sys/resource.h>
#endif

using namespace std;
using namespace __gnu_pbds;

#define sz(x) (int)((x).size())
#define all(x) (x).begin(), (x).end()
#define mem0(x) memset(x, 0, sizeof(x))
#define mem1(x) memset(x, -1, sizeof(x))
#define sleep(x) std::this_thread::sleep_for(std::chrono::milliseconds(x))
#define DECIMAL(n) std::cout << std::fixed << std::setprecision(n);
#define rep(i, a, b) for (int32_t i = (a); i < (b); ++i)
#define between(i, x, y) ((i) >= (x) && (i) <= (y))
// #define clamp(i,x,y)         clamp(i, x, y)
#define sqr(a) ((a) * (a))
#define pii pair<int32_t, int32_t>
#define pll pair<long long, long long>
#define mp make_pair
#define fi first
#define sc second
#define pb push_back
#define ppb pop_back
#define pf push_front
#define ppf pop_front
#define lbd lower_bound
#define ubd upper_bound
#define int long long
#define ll long long

template <typename T, typename U>
istream &operator>>(istream &in, pair<T, U> &a) {
  in >> a.first >> a.second;
  return in;
}
template <typename T, typename U>
ostream &operator<<(ostream &out, pair<T, U> a) {
  out << "{" << a.first << ", " << a.second << "}";
  return out;
}
template <typename T, typename U> static inline void remin(T &x, U y) {
  if (y < x) {
    x = y;
  }
}
template <typename T, typename U> static inline void remax(T &x, U y) {
  if (x < y) {
    x = y;
  }
}

static mt19937_64
    gen(chrono::duration_cast<chrono::nanoseconds>(
            chrono::high_resolution_clock::now().time_since_epoch())
            .count());

inline long long toint(const string &s) {
  stringstream ss;
  ss << s;
  long long x;
  ss >> x;
  return x;
}
inline string tostring(long long number) {
  stringstream ss;
  ss << number;
  return ss.str();
}
inline string tobin(long long x) { return bitset<63>(x).to_string(); }

void alllower(string &u) {
  transform(u.begin(), u.end(), u.begin(), ::tolower);
  return;
}
void allupper(string &u) {
  transform(u.begin(), u.end(), u.begin(), ::toupper);
  return;
}

template <typename T>
using ordered_set =
    tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
// find_by_order(k)  returns iterator to kth element starting from 0;
// order_of_key(k) returns count of elements strictly smaller than k;
// erase, insert same as normal set

const long double PI = 3.14159265358979323846264338;
const long double eps = 1e-12;
const long long fftmod = 998244353;
const long long MOD = 1e9 + 7;
const long long INF = 1e18;
const int32_t N = 1e5 + 10;
const int32_t NN = 1e6 + 10;

inline int inv(int a) {
  a %= MOD;
  if (a < 0)
    a += MOD;
  int b = MOD, u = 0, v = 1;
  while (a) {
    int t = b / a;
    b -= t * a;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  assert(b == 1);
  if (u < 0)
    u += MOD;
  return u;
}

int fact[N], invfact[N];

void init() {
  fact[0] = 1;
  int i;
  for (i = 1; i < N; i++) {
    fact[i] = (i * fact[i - 1]) % MOD;
  }
  i--;
  invfact[i] = inv(fact[i]);
  for (i--; i >= 0; i--) {
    invfact[i] = (invfact[i + 1] * (i + 1)) % MOD;
  }
}
int ncr(int n, int r) {
  if (n < r)
    return 0;
  return (((fact[n] * invfact[r]) % MOD) * invfact[n - r]) % MOD;
}

inline void dobbysolver(int testcase) {
  init();
  int h, w;
  cin >> h >> w;
  int a, b;
  cin >> a >> b;
  int ans = 0;
  for (int i = 1; i <= h - a; ++i) {
    ans += (ncr(i + b - 2, b - 1) * ncr(h + w - b - i - 1, w - b - 1)) % MOD;
    ans %= MOD;
  }
  cout << ans;

  return;
}

int32_t main() {
#ifdef on_linux
  rlimit cpu_time{.rlim_cur = 2, .rlim_max = RLIM_INFINITY};
  setrlimit(RLIMIT_CPU, &cpu_time);
#endif
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int TESTS = 1;
  // cin >> TESTS;
  for (int i = 1; i <= TESTS; ++i) {
    // cout << "Case #" << i << ": ";
    dobbysolver(i);
    if (i != TESTS)
      cout << '\n';
  }
  return 0;
}

Fixed submission:
// #pragma comment(linker, "/stack:200000000")
// #pragma GCC optimize("O3")
// #pragma GCC optimize("Ofast")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
// #pragma GCC optimize("unroll-loops")

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#ifdef on_linux
#include <sys/resource.h>
#endif

using namespace std;
using namespace __gnu_pbds;

#define sz(x) (int)((x).size())
#define all(x) (x).begin(), (x).end()
#define mem0(x) memset(x, 0, sizeof(x))
#define mem1(x) memset(x, -1, sizeof(x))
#define sleep(x) std::this_thread::sleep_for(std::chrono::milliseconds(x))
#define DECIMAL(n) std::cout << std::fixed << std::setprecision(n);
#define rep(i, a, b) for (int32_t i = (a); i < (b); ++i)
#define between(i, x, y) ((i) >= (x) && (i) <= (y))
// #define clamp(i,x,y)         clamp(i, x, y)
#define sqr(a) ((a) * (a))
#define pii pair<int32_t, int32_t>
#define pll pair<long long, long long>
#define mp make_pair
#define fi first
#define sc second
#define pb push_back
#define ppb pop_back
#define pf push_front
#define ppf pop_front
#define lbd lower_bound
#define ubd upper_bound
#define int long long
#define ll long long

template <typename T, typename U>
istream &operator>>(istream &in, pair<T, U> &a) {
  in >> a.first >> a.second;
  return in;
}
template <typename T, typename U>
ostream &operator<<(ostream &out, pair<T, U> a) {
  out << "{" << a.first << ", " << a.second << "}";
  return out;
}
template <typename T, typename U> static inline void remin(T &x, U y) {
  if (y < x) {
    x = y;
  }
}
template <typename T, typename U> static inline void remax(T &x, U y) {
  if (x < y) {
    x = y;
  }
}

static mt19937_64
    gen(chrono::duration_cast<chrono::nanoseconds>(
            chrono::high_resolution_clock::now().time_since_epoch())
            .count());

inline long long toint(const string &s) {
  stringstream ss;
  ss << s;
  long long x;
  ss >> x;
  return x;
}
inline string tostring(long long number) {
  stringstream ss;
  ss << number;
  return ss.str();
}
inline string tobin(long long x) { return bitset<63>(x).to_string(); }

void alllower(string &u) {
  transform(u.begin(), u.end(), u.begin(), ::tolower);
  return;
}
void allupper(string &u) {
  transform(u.begin(), u.end(), u.begin(), ::toupper);
  return;
}

template <typename T>
using ordered_set =
    tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
// find_by_order(k)  returns iterator to kth element starting from 0;
// order_of_key(k) returns count of elements strictly smaller than k;
// erase, insert same as normal set

const long double PI = 3.14159265358979323846264338;
const long double eps = 1e-12;
const long long fftmod = 998244353;
const long long MOD = 1e9 + 7;
const long long INF = 1e18;
const int32_t N = 5e5 + 10;
const int32_t NN = 1e6 + 10;

inline int inv(int a) {
  a %= MOD;
  if (a < 0)
    a += MOD;
  int b = MOD, u = 0, v = 1;
  while (a) {
    int t = b / a;
    b -= t * a;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  assert(b == 1);
  if (u < 0)
    u += MOD;
  return u;
}

int fact[N], invfact[N];

void init() {
  fact[0] = 1;
  int i;
  for (i = 1; i < N; i++) {
    fact[i] = (i * fact[i - 1]) % MOD;
  }
  i--;
  invfact[i] = inv(fact[i]);
  for (i--; i >= 0; i--) {
    invfact[i] = (invfact[i + 1] * (i + 1)) % MOD;
  }
}
int ncr(int n, int r) {
  if (n < r)
    return 0;
  return (((fact[n] * invfact[r]) % MOD) * invfact[n - r]) % MOD;
}

inline void dobbysolver(int testcase) {
  init();
  int h, w;
  cin >> h >> w;
  int a, b;
  cin >> a >> b;
  int ans = 0;
  for (int i = 1; i <= h - a; ++i) {
    ans += (ncr(i + b - 2, b - 1) * ncr(h + w - b - i - 1, w - b - 1)) % MOD;
    ans %= MOD;
  }
  cout << ans;

  return;
}

int32_t main() {
#ifdef on_linux
  rlimit cpu_time{.rlim_cur = 2, .rlim_max = RLIM_INFINITY};
  setrlimit(RLIMIT_CPU, &cpu_time);
#endif
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int TESTS = 1;
  // cin >> TESTS;
  for (int i = 1; i <= TESTS; ++i) {
    // cout << "Case #" << i << ": ";
    dobbysolver(i);
    if (i != TESTS)
      cout << '\n';
  }
  return 0;
}