Problem p02630 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>

using namespace std;
using ll = long long;
const int mod = 1e9 + 7;

struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}

  mint operator-() const { return mint(-x); }

  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }

  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }

  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }

  mint operator+(const mint a) const { return mint(*this) += a; }

  mint operator-(const mint a) const { return mint(*this) -= a; }

  mint operator*(const mint a) const { return mint(*this) *= a; }

  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }

  mint &operator/=(const mint a) { return *this *= a.inv(); }

  mint operator/(const mint a) const { return mint(*this) /= a; }
};

istream &operator>>(istream &is, const mint &a) { return is >> a.x; }

ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

struct combination {
  vector<mint> fact, ifact;

  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }

  mint operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};

struct UnionFind {
  vector<int> d;

  UnionFind(int n = 0) : d(n, -1) {}

  int find(int x) {
    if (d[x] < 0)
      return x;
    return d[x] = find(d[x]);
  }

  bool unite(int x, int y) {
    x = find(x);
    y = find(y);
    if (x == y)
      return false;
    if (d[x] > d[y])
      swap(x, y);
    d[x] += d[y];
    d[y] = x;
    return true;
  }

  bool same(int x, int y) { return find(x) == find(y); }

  int size(int x) { return -d[find(x)]; }
};

ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }

ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

int main() {
  ll n, q;
  cin >> n;
  vector<ll> B(1010101, 0);
  ll sum = 0;

  for (ll i = 0; i < n; ++i) {
    ll a;
    cin >> a;
    B[a]++;
    sum += a;
  }

  cin >> q;
  for (ll i = 0; i < q; ++i) {
    ll b, c;
    cin >> b >> c;
    sum -= B[b] * b;
    sum -= B[c] * c;
    B[c] += B[b];
    sum += B[c] * c;
    B[b] = 0;
    cout << sum << endl;
  }
  return 0;
}


Error: 0

Fixed submission:

#include <bits/stdc++.h>

using namespace std;
using ll = long long;
const int mod = 1e9 + 7;

struct mint {
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}

  mint operator-() const { return mint(-x); }

  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }

  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }

  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }

  mint operator+(const mint a) const { return mint(*this) += a; }

  mint operator-(const mint a) const { return mint(*this) -= a; }

  mint operator*(const mint a) const { return mint(*this) *= a; }

  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }

  mint &operator/=(const mint a) { return *this *= a.inv(); }

  mint operator/(const mint a) const { return mint(*this) /= a; }
};

istream &operator>>(istream &is, const mint &a) { return is >> a.x; }

ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

struct combination {
  vector<mint> fact, ifact;

  combination(int n) : fact(n + 1), ifact(n + 1) {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }

  mint operator()(int n, int k) {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};

struct UnionFind {
  vector<int> d;

  UnionFind(int n = 0) : d(n, -1) {}

  int find(int x) {
    if (d[x] < 0)
      return x;
    return d[x] = find(d[x]);
  }

  bool unite(int x, int y) {
    x = find(x);
    y = find(y);
    if (x == y)
      return false;
    if (d[x] > d[y])
      swap(x, y);
    d[x] += d[y];
    d[y] = x;
    return true;
  }

  bool same(int x, int y) { return find(x) == find(y); }

  int size(int x) { return -d[find(x)]; }
};

ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }

ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

set<string> A;
int H = 0;

void dfs(int k, string x) {
  if (k == 4) {
    sort(x.begin(), x.end());
    H++;
    A.insert(x);
    return;
  }
  for (int i = 0; i < 5; ++i) {
    char c = 'a' + i;
    x[k] = c;
    dfs(k + 1, x);
  }
}

int main() {
  int n, q;
  cin >> n;
  vector<int> A(n), B(n + 1);
  for (int i = 0; i < n; ++i) {
    cin >> A[i];
    B[A[i]]++;
  }
  cin >> q;
  ll sum = 0;
  for (int i = 0; i <= n; ++i) {
    sum += B[i] * i;
  }
  for (int i = 0; i < q; ++i) {
    int b, c;
    cin >> b >> c;
    sum -= B[b] * b + B[c] * c;
    B[c] += B[b];
    sum += B[c] * c;
    B[b] = 0;
    cout << sum << endl;
  }

  return 0;
}