Problem p02691 (C++) — buggy submission, verdict: Runtime Error

#include <algorithm>
#include <bitset>
#include <cctype>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <ctime>
#include <deque>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;
using ll = long long;

unsigned euclidean_gcd(unsigned a, unsigned b) {
  if (a < b)
    return euclidean_gcd(b, a);
  unsigned r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}

class UnionFind {
public:
  vector<ll> par;
  vector<ll> siz;
  UnionFind(ll sz_) : par(sz_), siz(sz_, 1LL) {
    for (ll i = 0; i < sz_; ++i)
      par[i] = i;
  }
  void init(ll sz_) {
    par.resize(sz_);
    siz.assign(sz_, 1LL);
    for (ll i = 0; i < sz_; ++i)
      par[i] = i;
  }
  ll root(ll x) {
    while (par[x] != x) {
      x = par[x] = par[par[x]];
    }
    return x;
  }
  bool merge(ll x, ll y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (siz[x] < siz[y])
      swap(x, y);
    siz[x] += siz[y];
    par[y] = x;
    return true;
  }

  bool issame(ll x, ll y) { return root(x) == root(y); }

  ll size(ll x) { return siz[root(x)]; }
};

long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

long long modinv(long long a, long long mod) { return modpow(a, mod - 2, mod); }

int main() {
  ll n;
  cin >> n;
  vector<ll> z(210100);
  vector<ll> x(210100);
  for (int i = 0; i < n; i++) {
    ll p;
    cin >> p;
    if (p + i < 210000)
      z.at(p + i)++;
    if (i - p < 210000 && i - p >= 0)
      x.at(i - p)++;
  }
  ll ans = 0;
  for (int i = 0; i < 200001; i++) {
    ans += z.at(i) * x.at(i);
  }
  cout << ans << endl;
}

Error: 0

Fixed submission:

#include <algorithm>
#include <bitset>
#include <cctype>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <ctime>
#include <deque>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;
using ll = long long;

unsigned euclidean_gcd(unsigned a, unsigned b) {
  if (a < b)
    return euclidean_gcd(b, a);
  unsigned r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}

class UnionFind {
public:
  vector<ll> par;
  vector<ll> siz;
  UnionFind(ll sz_) : par(sz_), siz(sz_, 1LL) {
    for (ll i = 0; i < sz_; ++i)
      par[i] = i;
  }
  void init(ll sz_) {
    par.resize(sz_);
    siz.assign(sz_, 1LL);
    for (ll i = 0; i < sz_; ++i)
      par[i] = i;
  }
  ll root(ll x) {
    while (par[x] != x) {
      x = par[x] = par[par[x]];
    }
    return x;
  }
  bool merge(ll x, ll y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (siz[x] < siz[y])
      swap(x, y);
    siz[x] += siz[y];
    par[y] = x;
    return true;
  }

  bool issame(ll x, ll y) { return root(x) == root(y); }

  ll size(ll x) { return siz[root(x)]; }
};

long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

long long modinv(long long a, long long mod) { return modpow(a, mod - 2, mod); }

int main() {
  ll n;
  cin >> n;
  vector<ll> z(210100);
  vector<ll> x(210100);
  for (int i = 0; i < n; i++) {
    ll p;
    cin >> p;
    if (p + i < 2100000)
      z.at(p + i)++;
    if (i - p < 2100000 && i - p >= 0)
      x.at(i - p)++;
  }
  ll ans = 0;
  for (int i = 0; i < 200000; i++) {
    ans += z.at(i) * x.at(i);
  }
  cout << ans << endl;
}