Problem p03837 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

const ll MAX_N = 110;
const ll INF = 1e12;

vector<pair<ll, ll>> g[MAX_N]; // firstに最短距離, secondに頂点番号
ll d[MAX_N];
int n; // 頂点数

bool used[MAX_N][MAX_N];

int pr[MAX_N]; // 最短路の直前の頂点

// bellman ford法で負辺がない場合。計算量はO(|E|log|V|)
void dijkstra(int s) {
  priority_queue<pair<ll, ll>, vector<pair<ll, ll>>, greater<pair<ll, ll>>> PQ;
  for (int i = 1; i <= n; i++)
    d[i] = INF;
  for (int i = 1; i <= n; i++)
    pr[i] = -1;
  d[s] = 0;
  PQ.push(make_pair(0, s));

  while (!PQ.empty()) {
    pair<ll, ll> p = PQ.top();
    PQ.pop();
    int v = p.second;
    if (d[v] < p.first)
      continue;
    for (auto gvi : g[v]) {
      ll cost = gvi.first;
      int u = gvi.second;
      if (d[u] > d[v] + cost) {
        d[u] = d[v] + cost;
        PQ.push(make_pair(d[u], u));
        pr[u] = v;
      }
    }
  }
}

// 頂点tへの最短路
void get_path(int t) {
  for (; t != -1; t = pr[t]) {
    used[t][pr[t]] = true;
    used[pr[t]][t] = true;
  }
}

int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);
  int m;
  cin >> n >> m;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      used[i][j] = true;
    }
  }
  for (int i = 0; i < m; i++) {
    ll a, b, c;
    cin >> a >> b >> c;
    g[a].push_back(make_pair(c, b));
    g[b].push_back(make_pair(c, a));
    used[a][b] = false;
    used[b][a] = false;
  }
  for (int i = 1; i <= n; i++) {
    dijkstra(i);
    for (int j = i + 1; j <= n; j++) {
      // cout << i << " " << j << " " << d[j] << endl;
      get_path(j);
    }
  }
  int cnt = 0;
  for (int i = 1; i <= n; i++) {
    for (int j = i + 1; j <= n; j++) {
      if (used[i][j] == false)
        cnt++;
    }
  }
  cout << cnt << endl;
}

Fixed submission:
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

const ll MAX_N = 1100;
const ll INF = 1e12;

vector<pair<ll, ll>> g[MAX_N]; // firstに最短距離, secondに頂点番号
ll d[MAX_N];
int n; // 頂点数

bool used[MAX_N][MAX_N];

int pr[MAX_N]; // 最短路の直前の頂点

// bellman ford法で負辺がない場合。計算量はO(|E|log|V|)
void dijkstra(int s) {
  priority_queue<pair<ll, ll>, vector<pair<ll, ll>>, greater<pair<ll, ll>>> PQ;
  for (int i = 1; i <= n; i++)
    d[i] = INF;
  for (int i = 1; i <= n; i++)
    pr[i] = -1;
  d[s] = 0;
  PQ.push(make_pair(0, s));

  while (!PQ.empty()) {
    pair<ll, ll> p = PQ.top();
    PQ.pop();
    int v = p.second;
    if (d[v] < p.first)
      continue;
    for (auto gvi : g[v]) {
      ll cost = gvi.first;
      int u = gvi.second;
      if (d[u] > d[v] + cost) {
        d[u] = d[v] + cost;
        PQ.push(make_pair(d[u], u));
        pr[u] = v;
      }
    }
  }
}

// 頂点tへの最短路
void get_path(int t) {
  for (; t != -1; t = pr[t]) {
    used[t][pr[t]] = true;
    used[pr[t]][t] = true;
  }
}

int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);
  int m;
  cin >> n >> m;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
      used[i][j] = true;
    }
  }
  for (int i = 0; i < m; i++) {
    ll a, b, c;
    cin >> a >> b >> c;
    g[a].push_back(make_pair(c, b));
    g[b].push_back(make_pair(c, a));
    used[a][b] = false;
    used[b][a] = false;
  }
  for (int i = 1; i <= n; i++) {
    dijkstra(i);
    for (int j = i + 1; j <= n; j++) {
      // cout << i << " " << j << " " << d[j] << endl;
      get_path(j);
    }
  }
  int cnt = 0;
  for (int i = 1; i <= n; i++) {
    for (int j = i + 1; j <= n; j++) {
      if (used[i][j] == false)
        cnt++;
    }
  }
  cout << cnt << endl;
}