Problem p02840 (C++)

Buggy submission (status: Runtime Error):
// #pragma GCC target("avx")  // CPU 処理並列化
// #pragma GCC optimize("O3")  // CPU 処理並列化
// #pragma GCC optimize("unroll-loops")  // 条件処理の呼び出しを減らす
#include <algorithm>
#include <bitset>
#include <cassert>
#include <complex>
#include <deque>
#include <map>
#include <math.h>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <string>
#include <time.h>
#include <unordered_set>
#include <vector>
using namespace std;
const long long mod = 1000000007;
const long long inf = mod * mod;
const long long d2 = (mod + 1) / 2;
const double EPS = 1e-10;
const double INF = 1e+10;
const double PI = acos(-1.0);
const int C_SIZE = 3100000;
const int UF_SIZE = 3100000;
namespace {
long long fact[C_SIZE];
long long finv[C_SIZE];
long long inv[C_SIZE];
long long Comb(int a, int b) {
  if (a < b || b < 0)
    return 0;
  return fact[a] * finv[b] % mod * finv[a - b] % mod;
}
void init_C(int n) {
  fact[0] = finv[0] = inv[1] = 1;
  for (int i = 2; i < n; i++) {
    inv[i] = (mod - (mod / i) * inv[mod % i] % mod) % mod;
  }
  for (int i = 1; i < n; i++) {
    fact[i] = fact[i - 1] * i % mod;
    finv[i] = finv[i - 1] * inv[i] % mod;
  }
}
long long pw(long long a, long long b) {
  if (a < 0LL)
    return 0;
  if (b < 0LL)
    return 0;
  long long ret = 1;
  while (b) {
    if (b % 2)
      ret = ret * a % mod;
    a = a * a % mod;
    b /= 2;
  }
  return ret;
}
long long pw_mod(long long a, long long b, long long M) {
  if (a < 0LL)
    return 0;
  if (b < 0LL)
    return 0;
  long long ret = 1;
  while (b) {
    if (b % 2)
      ret = ret * a % M;
    a = a * a % M;
    b /= 2;
  }
  return ret;
}
int pw_mod_int(int a, int b, int M) {
  if (a < 0)
    return 0;
  if (b < 0)
    return 0;
  int ret = 1;
  while (b) {
    if (b % 2)
      ret = (long long)ret * a % M;
    a = (long long)a * a % M;
    b /= 2;
  }
  return ret;
}
int ABS(int a) { return max(a, -a); }
long long ABS(long long a) { return max(a, -a); }
double ABS(double a) { return max(a, -a); }
int sig(double r) { return (r < -EPS) ? -1 : (r > +EPS) ? +1 : 0; }
int UF[UF_SIZE];
void init_UF(int n) {
  for (int i = 0; i < n; i++)
    UF[i] = -1;
}
int FIND(int a) {
  if (UF[a] < 0)
    return a;
  return UF[a] = FIND(UF[a]);
}
void UNION(int a, int b) {
  a = FIND(a);
  b = FIND(b);
  if (a == b)
    return;
  if (UF[a] > UF[b])
    swap(a, b);
  UF[a] += UF[b];
  UF[b] = a;
}
} // namespace
// ここから編集しろ
pair<long long, pair<long long, int>> ev[410000];
long long t[210000];
long long L[210000];
long long R[210000];
int main() {
  long long a, b, c;
  scanf("%lld%lld%lld", &a, &b, &c);
  for (int i = 0; i < a; i++) {
    t[i] = b + c * i;
  }
  for (int i = 0; i < a; i++) {
    L[i + 1] = L[i] + t[i];
    R[i + 1] = R[i] + t[a - 1 - i];
  }
  long long ret = 0;
  for (int i = 0; i <= a; i++) {
    long long left = (L[i] - R[a - i]);
    long long right = (R[i] - L[a - i]);
    if (left > right)
      swap(left, right);
    ev[i * 2] = make_pair((left % ABS(c * 2) + ABS(c * 2)) % ABS(c * 2),
                          make_pair(left, 1));
    ev[i * 2 + 1] = make_pair((right % ABS(c * 2) + ABS(c * 2)) % ABS(c * 2),
                              make_pair(right + ABS(c * 2), -1));
    // printf("%d: %lld %lld
    // %lld\n",(left%ABS(c*2)+ABS(c*2))%ABS(c*2),left,right+ABS(c*2));
  }
  std::sort(ev, ev + a * 2 + 2);
  int cur = 0;
  for (int i = 0; i < a * 2 + 2; i++) {
    if (ev[i].second.second == 1)
      cur++;
    else
      cur--;
    if (cur > 0) {
      ret += (ev[i + 1].second.first - ev[i].second.first) / ABS(c * 2);
    }
  }
  printf("%lld\n", ret);
}

Fixed submission:
// #pragma GCC target("avx")  // CPU 処理並列化
// #pragma GCC optimize("O3")  // CPU 処理並列化
// #pragma GCC optimize("unroll-loops")  // 条件処理の呼び出しを減らす
#include <algorithm>
#include <bitset>
#include <cassert>
#include <complex>
#include <deque>
#include <map>
#include <math.h>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <string>
#include <time.h>
#include <unordered_set>
#include <vector>
using namespace std;
const long long mod = 1000000007;
const long long inf = mod * mod;
const long long d2 = (mod + 1) / 2;
const double EPS = 1e-10;
const double INF = 1e+10;
const double PI = acos(-1.0);
const int C_SIZE = 3100000;
const int UF_SIZE = 3100000;
namespace {
long long fact[C_SIZE];
long long finv[C_SIZE];
long long inv[C_SIZE];
long long Comb(int a, int b) {
  if (a < b || b < 0)
    return 0;
  return fact[a] * finv[b] % mod * finv[a - b] % mod;
}
void init_C(int n) {
  fact[0] = finv[0] = inv[1] = 1;
  for (int i = 2; i < n; i++) {
    inv[i] = (mod - (mod / i) * inv[mod % i] % mod) % mod;
  }
  for (int i = 1; i < n; i++) {
    fact[i] = fact[i - 1] * i % mod;
    finv[i] = finv[i - 1] * inv[i] % mod;
  }
}
long long pw(long long a, long long b) {
  if (a < 0LL)
    return 0;
  if (b < 0LL)
    return 0;
  long long ret = 1;
  while (b) {
    if (b % 2)
      ret = ret * a % mod;
    a = a * a % mod;
    b /= 2;
  }
  return ret;
}
long long pw_mod(long long a, long long b, long long M) {
  if (a < 0LL)
    return 0;
  if (b < 0LL)
    return 0;
  long long ret = 1;
  while (b) {
    if (b % 2)
      ret = ret * a % M;
    a = a * a % M;
    b /= 2;
  }
  return ret;
}
int pw_mod_int(int a, int b, int M) {
  if (a < 0)
    return 0;
  if (b < 0)
    return 0;
  int ret = 1;
  while (b) {
    if (b % 2)
      ret = (long long)ret * a % M;
    a = (long long)a * a % M;
    b /= 2;
  }
  return ret;
}
int ABS(int a) { return max(a, -a); }
long long ABS(long long a) { return max(a, -a); }
double ABS(double a) { return max(a, -a); }
int sig(double r) { return (r < -EPS) ? -1 : (r > +EPS) ? +1 : 0; }
int UF[UF_SIZE];
void init_UF(int n) {
  for (int i = 0; i < n; i++)
    UF[i] = -1;
}
int FIND(int a) {
  if (UF[a] < 0)
    return a;
  return UF[a] = FIND(UF[a]);
}
void UNION(int a, int b) {
  a = FIND(a);
  b = FIND(b);
  if (a == b)
    return;
  if (UF[a] > UF[b])
    swap(a, b);
  UF[a] += UF[b];
  UF[b] = a;
}
} // namespace
// ここから編集しろ
pair<long long, pair<long long, int>> ev[410000];
long long t[210000];
long long L[210000];
long long R[210000];
int main() {
  long long a, b, c;
  scanf("%lld%lld%lld", &a, &b, &c);
  if (c == 0) {
    if (b == 0) {
      printf("1\n");
      return 0;
    }
    printf("%lld\n", a + 1);
    return 0;
  }
  for (int i = 0; i < a; i++) {
    t[i] = b + c * i;
  }
  for (int i = 0; i < a; i++) {
    L[i + 1] = L[i] + t[i];
    R[i + 1] = R[i] + t[a - 1 - i];
  }
  long long ret = 0;
  for (int i = 0; i <= a; i++) {
    long long left = (L[i] - R[a - i]);
    long long right = (R[i] - L[a - i]);
    if (left > right)
      swap(left, right);
    ev[i * 2] = make_pair((left % ABS(c * 2) + ABS(c * 2)) % ABS(c * 2),
                          make_pair(left, 1));
    ev[i * 2 + 1] = make_pair((right % ABS(c * 2) + ABS(c * 2)) % ABS(c * 2),
                              make_pair(right + ABS(c * 2), -1));
    // printf("%d: %lld %lld
    // %lld\n",(left%ABS(c*2)+ABS(c*2))%ABS(c*2),left,right+ABS(c*2));
  }
  std::sort(ev, ev + a * 2 + 2);
  int cur = 0;
  for (int i = 0; i < a * 2 + 2; i++) {
    if (ev[i].second.second == 1)
      cur++;
    else
      cur--;
    if (cur > 0) {
      ret += (ev[i + 1].second.first - ev[i].second.first) / ABS(c * 2);
    }
  }
  printf("%lld\n", ret);
}