Problem p02689 (C++) — buggy submission, verdict: Memory Limit Exceeded

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

using ll = long long;

template <typename T> T gcd(T a, T b) {
  if (a < b)
    return gcd(b, a);
  T r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}

template <typename T> T lcm(T a, T b) { return a * b / gcd(a, b); }

template <typename T> T ext_gcd(T a, T b, T &x, T &y) {
  x = 1;
  y = 0;
  T x0 = 0, y0 = 1;
  while (b != 0) {
    T q = a / b;
    a -= q * b;
    swap(a, b);
    x -= q * x0;
    swap(x, x0);
    y -= q * y0;
    swap(y, y0);
  }
  return a;
}

template <typename T> T inv_mod(T a, T mod) {
  T t1, t2;
  ext_gcd(a, mod, t1, t2);
  t1 %= mod;
  if (t1 < 0)
    t1 += mod;
  return t1;
}

template <typename T> T combination_large(T n, T k, T mod) {
  if (n < k || n < 0 || k < 0)
    return 0;
  vector<T> inv(n + 1, 1), fact(n + 1, 1), fact_inv(n + 1, 1);
  for (T i = 2; i <= n; ++i) {
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    fact[i] = (fact[i - 1] * i) % mod;
    fact_inv[i] = fact_inv[i - 1] * inv[i] % mod;
  }
  return fact[n] * (fact_inv[k] * fact_inv[n - k] % mod) % mod;
}

template <typename T> T combination(T n, T k, T mod = 1) {
  vector<vector<T>> table(n + 1, vector<T>(k + 1, 1));
  for (T i = 1; i <= n; ++i) {
    table[i][0] = 1;
    for (T j = 1; j <= n; ++j) {
      table[i][j] = (table[i - 1][j - 1] + table[i - 1][j]) % mod;
    }
  }
}

int main(int, char **) {
  long long N;
  scanf("%lld", &N);
  long long M;
  scanf("%lld", &M);
  std::vector<long long> H(N);
  for (ll i = 0; i < N; i++) {
    scanf("%lld", &H[i]);
  }
  unordered_set<ll> s;
  vector<ll> g(N);
  vector<int> good(N, 0);
  for (ll i = 0; i < N; ++i) {
    g[i] = i;
  }
  for (ll i = 0; i < M; i++) {
    ll a, b;
    cin >> a >> b;
    s.insert(a - 1);
    s.insert(b - 1);
    if (H[a - 1] > H[b - 1]) {
      g[b - 1] = a;
      good[b - 1] = -1;
    } else if (H[a - 1] < H[b - 1]) {
      g[a - 1] = b;
      good[a - 1] = -1;
    } else {
      good[a - 1] = -1;
      good[b - 1] = -1;
    }
  }
  for (ll i = 0; i < N; ++i) {
    if (s.find(i) == s.end()) {
      good[i] = 1;
    }
  }

  for (ll i = 0; i < N; ++i) {
    if (good[i] < 0)
      continue;
    if (g[i] == i) {
      good[i] = 1;
    }
  }

  ll sum = 0;
  for (ll i = 0; i < N; ++i) {
    if (good[i] > 0) {
      // cerr << i << endl;
      ++sum;
    }
  }
  cout << sum << endl;

  return 0;
}


Error: MLE

Fixed submission:

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

using ll = long long;

template <typename T> T gcd(T a, T b) {
  if (a < b)
    return gcd(b, a);
  T r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}

template <typename T> T lcm(T a, T b) { return a * b / gcd(a, b); }

template <typename T> T ext_gcd(T a, T b, T &x, T &y) {
  x = 1;
  y = 0;
  T x0 = 0, y0 = 1;
  while (b != 0) {
    T q = a / b;
    a -= q * b;
    swap(a, b);
    x -= q * x0;
    swap(x, x0);
    y -= q * y0;
    swap(y, y0);
  }
  return a;
}

template <typename T> T inv_mod(T a, T mod) {
  T t1, t2;
  ext_gcd(a, mod, t1, t2);
  t1 %= mod;
  if (t1 < 0)
    t1 += mod;
  return t1;
}

template <typename T> T combination_large(T n, T k, T mod) {
  if (n < k || n < 0 || k < 0)
    return 0;
  vector<T> inv(n + 1, 1), fact(n + 1, 1), fact_inv(n + 1, 1);
  for (T i = 2; i <= n; ++i) {
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    fact[i] = (fact[i - 1] * i) % mod;
    fact_inv[i] = fact_inv[i - 1] * inv[i] % mod;
  }
  return fact[n] * (fact_inv[k] * fact_inv[n - k] % mod) % mod;
}

template <typename T> T combination(T n, T k, T mod = 1) {
  vector<vector<T>> table(n + 1, vector<T>(k + 1, 1));
  for (T i = 1; i <= n; ++i) {
    table[i][0] = 1;
    for (T j = 1; j <= n; ++j) {
      table[i][j] = (table[i - 1][j - 1] + table[i - 1][j]) % mod;
    }
  }
}

int main(int, char **) {
  long long N;
  scanf("%lld", &N);
  long long M;
  scanf("%lld", &M);
  std::vector<long long> H(N);
  for (ll i = 0; i < N; i++) {
    scanf("%lld", &H[i]);
  }
  vector<vector<bool>> adj(N, vector<bool>(N, false));
  unordered_set<ll> s;
  vector<ll> g(N);
  vector<int> good(N, 0);
  for (ll i = 0; i < N; ++i) {
    g[i] = i;
  }
  for (ll i = 0; i < M; i++) {
    ll a, b;
    cin >> a >> b;
    adj[a - 1][b - 1] = true;
    adj[b - 1][a - 1] = true;
    s.insert(a - 1);
    s.insert(b - 1);
    if (H[a - 1] > H[b - 1]) {
      g[b - 1] = a;
      good[b - 1] = -1;
    } else if (H[a - 1] < H[b - 1]) {
      g[a - 1] = b;
      good[a - 1] = -1;
    } else {
      good[a - 1] = -1;
      good[b - 1] = -1;
    }
  }
  for (ll i = 0; i < N; ++i) {
    if (s.find(i) == s.end()) {
      good[i] = 1;
    }
  }

  for (ll i = 0; i < N; ++i) {
    if (good[i] < 0)
      continue;
    if (g[i] == i) {
      good[i] = 1;
    }
  }

  ll sum = 0;
  for (ll i = 0; i < N; ++i) {
    if (good[i] > 0) {
      // cerr << i << endl;
      ++sum;
    }
  }
  cout << sum << endl;

  return 0;
}