Problem p03184 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cmath>
#include <complex>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <regex>
#include <set>
#include <stack>
#include <string>
#include <vector>

const int MOD = 1e9 + 7;
const int iINF = 2147483647 / 2;
const long long int llINF = 9223372036854775807 / 2;

using namespace std;
using ll = long long int;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using vvvl = vector<vector<vector<ll>>>;

typedef pair<ll, ll> pll;
bool paircomp(const pll &a, const pll &b) {
  if (a.first == b.first)
    return a.second < b.second;
  return a.first < b.first;
}

#define REP(i, n) for (ll i = 0; i < (n); i++)
#define RREP(i, n) for (ll i = (n)-1; i >= 0; i--)
#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define AUTO(i, m) for (auto &i : m)
#define ALL(a) (a).begin(), (a).end()
#define MAX(vec) *std::max_element(vec.begin(), vec.end())
#define MIN(vec) *std::min_element(vec.begin(), vec.end())
#define ARGMAX(vec)                                                            \
  std::distance(vec.begin(), std::max_element(vec.begin(), vec.end()))
#define ARGMIN(vec)                                                            \
  std::distance(vec.begin(), std::min_element(vec.begin(), vec.end()))
#define REV(T) greater<T>()
#define PQ(T) priority_queue<T, vector<T>, greater<T>>
#define VVL(a, b, c) vector<vector<ll>>(a, vector<ll>(b, c))
#define VV(T, a, b, c) vector<vector<T>>(a, vector<T>(b, c))
#define VVVL(a, b, c, d)                                                       \
  vector<vector<vector<ll>>>(a, vector<vector<ll>>(b, vector<ll>(c, d)))
#define VVV(T, a, b, c, d)                                                     \
  vector<vector<vector<T>>>(a, vector<vector<T>>(b, vector<T>(c, d)))
#define SP(a) setprecision(a)
#define SQRT(a) sqrt((long double)(a))
#define DPOW(a, b) pow((long double)(a), (long double)(b))
#define UNIQUE(vec)                                                            \
  do {                                                                         \
    sort(ALL((vec)));                                                          \
    (vec).erase(std::unique(ALL((vec))), (vec).end());                         \
  } while (0)

ll MODPOW(ll n, ll m) {
  if (m == 0) {
    return 1;
  } else if (m % 2 == 0) {
    ll tmp = MODPOW(n, m / 2);
    return (tmp * tmp) % MOD;
  } else {
    return (n * MODPOW(n, m - 1)) % MOD;
  }
}

ll MODDIV(ll a, ll b) { return a * MODPOW(b, MOD - 2) % MOD; }
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ll H, W, N;
  cin >> H >> W >> N;
  vl r(N + 2), c(N + 2);
  r[0] = 1;
  c[0] = 1;
  REP(i, N) cin >> r[i + 1] >> c[i + 1];
  r[N + 1] = H;
  c[N + 1] = W;

  ll start = 0;
  ll goal = N + 1;
  vvl reach(N + 2);
  vl fact(H + W + 1, 0);
  fact[0] = 1;
  REP(i, H + W + 1) fact[i + 1] = ((i + 1) * fact[i]) % MOD;
  REP(i, N + 2) REP(j, N + 2) {
    if (i == j)
      continue;
    if (r[i] <= r[j] && c[i] <= c[j])
      reach[i].push_back(j);
  }
  vector<pll> order;
  REP(i, N + 2) order.emplace_back(r[i] + c[i], i);
  sort(ALL(order));
  vvl dp = VVL(N + 2, 2, 0);
  dp[0][0] = 1;
  REP(i, N + 2) {
    ll u = order[i].second;
    AUTO(v, reach[u]) {
      ll rdif = r[v] - r[u];
      ll cdif = c[v] - c[u];
      ll mag = MODDIV(fact[rdif + cdif], (fact[rdif] * fact[cdif]) % MOD);
      REP(p, 2) {
        ll base = dp[u][p];
        if (base == 0)
          continue;
        dp[v][1 - p] = (dp[v][1 - p] + (dp[u][p] * mag) % MOD) % MOD;
      }
    }
  }
  ll ans = (MOD + dp[goal][1] - dp[goal][0]) % MOD;
  cout << ans << endl;

  return 0;
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <cmath>
#include <complex>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <regex>
#include <set>
#include <stack>
#include <string>
#include <vector>

const int MOD = 1e9 + 7;
const int iINF = 2147483647 / 2;
const long long int llINF = 9223372036854775807 / 2;

using namespace std;
using ll = long long int;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using vvvl = vector<vector<vector<ll>>>;

typedef pair<ll, ll> pll;
bool paircomp(const pll &a, const pll &b) {
  if (a.first == b.first)
    return a.second < b.second;
  return a.first < b.first;
}

#define REP(i, n) for (ll i = 0; i < (n); i++)
#define RREP(i, n) for (ll i = (n)-1; i >= 0; i--)
#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define AUTO(i, m) for (auto &i : m)
#define ALL(a) (a).begin(), (a).end()
#define MAX(vec) *std::max_element(vec.begin(), vec.end())
#define MIN(vec) *std::min_element(vec.begin(), vec.end())
#define ARGMAX(vec)                                                            \
  std::distance(vec.begin(), std::max_element(vec.begin(), vec.end()))
#define ARGMIN(vec)                                                            \
  std::distance(vec.begin(), std::min_element(vec.begin(), vec.end()))
#define REV(T) greater<T>()
#define PQ(T) priority_queue<T, vector<T>, greater<T>>
#define VVL(a, b, c) vector<vector<ll>>(a, vector<ll>(b, c))
#define VV(T, a, b, c) vector<vector<T>>(a, vector<T>(b, c))
#define VVVL(a, b, c, d)                                                       \
  vector<vector<vector<ll>>>(a, vector<vector<ll>>(b, vector<ll>(c, d)))
#define VVV(T, a, b, c, d)                                                     \
  vector<vector<vector<T>>>(a, vector<vector<T>>(b, vector<T>(c, d)))
#define SP(a) setprecision(a)
#define SQRT(a) sqrt((long double)(a))
#define DPOW(a, b) pow((long double)(a), (long double)(b))
#define UNIQUE(vec)                                                            \
  do {                                                                         \
    sort(ALL((vec)));                                                          \
    (vec).erase(std::unique(ALL((vec))), (vec).end());                         \
  } while (0)

ll MODPOW(ll n, ll m) {
  if (m == 0) {
    return 1;
  } else if (m % 2 == 0) {
    ll tmp = MODPOW(n, m / 2);
    return (tmp * tmp) % MOD;
  } else {
    return (n * MODPOW(n, m - 1)) % MOD;
  }
}

ll MODDIV(ll a, ll b) { return a * MODPOW(b, MOD - 2) % MOD; }
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ll H, W, N;
  cin >> H >> W >> N;
  vl r(N + 2), c(N + 2);
  r[0] = 1;
  c[0] = 1;
  REP(i, N) cin >> r[i + 1] >> c[i + 1];
  r[N + 1] = H;
  c[N + 1] = W;

  ll start = 0;
  ll goal = N + 1;
  vvl reach(N + 2);
  vl fact(H + W + 1, 0);
  fact[0] = 1;
  REP(i, H + W) fact[i + 1] = ((i + 1) * fact[i]) % MOD;
  REP(i, N + 2) REP(j, N + 2) {
    if (i == j)
      continue;
    if (r[i] <= r[j] && c[i] <= c[j])
      reach[i].push_back(j);
  }
  vector<pll> order;
  REP(i, N + 2) order.emplace_back(r[i] + c[i], i);
  sort(ALL(order));
  vvl dp = VVL(N + 2, 2, 0);
  dp[0][0] = 1;
  REP(i, N + 2) {
    ll u = order[i].second;
    AUTO(v, reach[u]) {
      ll rdif = r[v] - r[u];
      ll cdif = c[v] - c[u];
      ll mag = MODDIV(fact[rdif + cdif], (fact[rdif] * fact[cdif]) % MOD);
      REP(p, 2) {
        ll base = dp[u][p];
        if (base == 0)
          continue;
        dp[v][1 - p] = (dp[v][1 - p] + (dp[u][p] * mag) % MOD) % MOD;
      }
    }
  }
  ll ans = (MOD + dp[goal][1] - dp[goal][0]) % MOD;
  cout << ans << endl;

  return 0;
}