Problem p02617 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
#define int long long // [9,223,372,036,854,775,807 to -9.....808] 19 digits
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define mp make_pair
#define vi vector<int>
#define vpi vector<pair<int, int>>
#define mii map<int, int>
#define x first
#define y second
#define mem(a, x) memset(a, x, sizeof(a))
#define rep(i, a, b) for (int i = a; i < b; i++)
#define sz(x) (int)((x).size())

typedef tree<int, null_type, less<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    ordered_set; // find_by_order()  // order_of_key

// const int32_t hellhsh = 444440583533; const double pi=2*acos(0.0);
const int32_t mod = 1e9 + 7;
const int32_t maxn = 2e5 + 5;

void solve() {
  int n;
  cin >> n;
  int ans = 0;
  rep(i, 1, n) {
    int u, v;
    cin >> u >> v;
    if (u > v)
      swap(u, v);
    ans += -1 * (u) * (n - v + 1);
  }
  rep(i, 0, n) ans += (i + 1) * (n - i);
  cout << ans;
}

signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int tc = 1, k = 1;
  // cin>>tc;
  while (tc--) {
    // cout<<"Case #"<<k++<<": ";
    solve();
  }
  return 0;
}

Error: WA

Fixed submission:

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
#define int long long // [9,223,372,036,854,775,807 to -9.....808] 19 digits
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define mp make_pair
#define vi vector<int>
#define vpi vector<pair<int, int>>
#define mii map<int, int>
#define x first
#define y second
#define mem(a, x) memset(a, x, sizeof(a))
#define rep(i, a, b) for (int i = a; i < b; i++)
#define sz(x) (int)((x).size())

typedef tree<int, null_type, less<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    ordered_set;

// const int32_t MOD=163577857; const int32_t hellhsh = 444440583533; const
// double pi=2*acos(0.0); const int32_t MOD=998244353;
const int32_t MOD = 1e9 + 7;
const int32_t maxn = 2e5 + 5;

struct mod_int {
  int32_t val;
  mod_int(long long v = 0) {
    if (v < 0)
      v = v % MOD + MOD;
    if (v >= MOD)
      v %= MOD;
    val = v;
  }

  static int32_t mod_inv(int32_t a, int32_t m = MOD) {
    int32_t g = m, r = a, x = 0, y = 1;
    while (r != 0) {
      int32_t q = g / r;
      g %= r;
      swap(g, r);
      x -= q * y;
      swap(x, y);
    }
    return x < 0 ? x + m : x;
  }
  static unsigned fast_mod(uint64_t x, unsigned m = MOD) {
#if !defined(_WIN32) || defined(_WIN64)
    return x % m;
#endif
    unsigned x_high = x >> 32, x_low = (unsigned)x;
    unsigned quot, rem;
    asm("divl %4\\n" : "=a"(quot), "=d"(rem) : "d"(x_high), "a"(x_low), "r"(m));
    return rem;
  }

  explicit operator int() const { return val; }
  mod_int &operator+=(const mod_int &other) {
    val += other.val;
    if (val >= MOD)
      val -= MOD;
    return *this;
  }
  mod_int &operator-=(const mod_int &other) {
    val -= other.val;
    if (val < 0)
      val += MOD;
    return *this;
  }
  mod_int &operator*=(const mod_int &other) {
    val = fast_mod((uint64_t)val * other.val);
    return *this;
  }
  mod_int &operator/=(const mod_int &other) { return *this *= other.inv(); }
  friend mod_int operator+(const mod_int &a, const mod_int &b) {
    return mod_int(a) += b;
  }
  friend mod_int operator-(const mod_int &a, const mod_int &b) {
    return mod_int(a) -= b;
  }
  friend mod_int operator*(const mod_int &a, const mod_int &b) {
    return mod_int(a) *= b;
  }
  friend mod_int operator/(const mod_int &a, const mod_int &b) {
    return mod_int(a) /= b;
  }
  mod_int &operator++() {
    val = val == MOD - 1 ? 0 : val + 1;
    return *this;
  }
  mod_int &operator--() {
    val = val == 0 ? MOD - 1 : val - 1;
    return *this;
  }
  mod_int operator++(int32_t) {
    mod_int before = *this;
    ++*this;
    return before;
  }
  mod_int operator--(int32_t) {
    mod_int before = *this;
    --*this;
    return before;
  }
  mod_int operator-() const { return val == 0 ? 0 : MOD - val; }
  bool operator==(const mod_int &other) const { return val == other.val; }
  bool operator!=(const mod_int &other) const { return val != other.val; }
  mod_int inv() const { return mod_inv(val); }
  friend ostream &operator<<(ostream &stream, const mod_int &m) {
    return stream << m.val;
  }
  friend istream &operator>>(istream &stream, mod_int &m) {
    return stream >> m.val;
  }

  mod_int pow(long long p) const { // dont put a negetive p
    assert(p >= 0);
    mod_int a = *this, result = 1;
    while (p > 0) {
      if (p & 1)
        result *= a;

      a *= a;
      p >>= 1;
    }
    return result;
  }
};
mod_int fact[maxn];
bool fbb = 0;
void build() {
  fbb = 1;
  fact[0] = 1;
  rep(i, 1, maxn) fact[i] = i * fact[i - 1];
}
mod_int C(int n, int r) {
  if (fbb)
    return (fact[n] / (fact[r] * fact[n - r]));
  mod_int a = 1, b = 1;
  rep(i, 0, r) {
    a *= n - i;
    b *= i + 1;
  }
  return a / b;
}

void solve() {
  int n, k;
  cin >> n >> k;
  deque<int> pos, neg;
  vi v1, v2;
  rep(i, 0, n) {
    int x;
    cin >> x;
    if (x < 0)
      v1.push_back(-x);
    else
      v2.push_back(x);
  }
  sort(all(v1));
  sort(all(v2));
  for (int i : v1)
    neg.push_back(i);
  for (int i : v2)
    pos.push_back(i);

  mod_int ans = 1;
  while (k) {
    bool bo = 0;
    if (sz(pos)) {
      if (sz(neg) < 2 or sz(pos) < 2 or k < 2) {
        ans *= pos.back();
        pos.pop_back();
        k--;
      } else {
        int n1 = neg.back();
        neg.pop_back();
        int n2 = neg.back();
        neg.pop_back();
        int p1 = pos.back();
        pos.pop_back();
        int p2 = pos.back();
        pos.pop_back();

        int nn = n1 * n2, pp = p1 * p2;

        if (nn > pp) {
          ans *= n1 * n2;
          pos.push_back(p2);
          pos.push_back(p1);
        } else {
          ans *= p1 * p2;
          neg.push_back(n2);
          neg.push_back(n1);
        }
        k -= 2;
      }
    } else {
      if (k % 2) {
        while (k) {
          ans *= -neg.front();
          neg.pop_front();
          k--;
        }
      } else {
        while (k) {
          ans *= neg.back();
          neg.pop_back();
          k--;
        }
      }
    }
  }
  cout << ans;
}

signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int tc = 1, k = 1;
  // cin>>tc;
  while (tc--) {
    // cout<<"Case #"<<k++<<": ";
    solve();
  }
  return 0;
}