Problem p02631 (C++) — buggy submission, verdict: Runtime Error

#define _CRT_SECURE_NO_WARNINGS
#pragma comment(linker, "/stack:512000000")
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <ccomplex>
#include <chrono>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#ifdef ONLINE_JUDGE
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,tune=native")
#pragma GCC optimize("unroll-loops")
#define int128 __int128
#else
#define int128 long long
#endif

#define all(x) x.begin(), x.end()
#define make_unique(x)                                                         \\
  sort(all((x)));                                                              \\
  (x).resize(unique(all((x))) - (x).begin())
#define mp make_pair
#define X first
#define Y second

using namespace std;

using ll = long long;
using ull = unsigned long long;
using ld = long double;
mt19937_64 gen(time(nullptr));

struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    // http://xorshift.di.unimi.it/splitmix64.c
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }

  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};

ll mod = 1e9 + 7;

namespace {
ll mul(ll a, ll b) {
  ll val = a * b - (ll)((ld)a * b / mod) * mod;
  if (val < 0)
    val += mod;
  if (val >= mod)
    val -= mod;
  return val;
}
ll poww(ll a, ll b) {
  ll val = 1;
  a %= mod;
  while (b > 0) {
    if (b % 2)
      val = mul(a, val);
    a = mul(a, a);
    b >>= 1;
  }
  return val % mod;
}
ll inv(ll a) { return poww(a, mod - 2); }
} // namespace

ll const maxn = 3e5 + 5;

signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  // freopen("output.txt", "w", stdout);
  ll n;
  cin >> n;
  vector<ll> a(n);
  ll sm = 0;
  for (ll i = 0; i < n; i++)
    cin >> a[i], sm ^= a[i];
  vector<ll> b(n);
  for (ll i = 0; i < n; i++) {
    b[i] = (sm ^ a[i]);
    // b[i] = (a[i - 1] ^ a[i]) ^ b[i - 1];
  }
  sm = 0;
  for (ll i = 0; i < n; i++)
    sm ^= b[i];
  for (ll i = 0; i < n; i++) {
    assert(a[i] == (sm ^ (b[i])));
  }
  for (ll i = 0; i < n; i++) {
    cout << b[i] << " ";
  }

  return 0;
}

/*

*/

Error: WA

Fixed submission:

#define _CRT_SECURE_NO_WARNINGS
#pragma comment(linker, "/stack:512000000")
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <ccomplex>
#include <chrono>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#ifdef ONLINE_JUDGE
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,tune=native")
#pragma GCC optimize("unroll-loops")
#define int128 __int128
#else
#define int128 long long
#endif

#define all(x) x.begin(), x.end()
#define make_unique(x)                                                         \\
  sort(all((x)));                                                              \\
  (x).resize(unique(all((x))) - (x).begin())
#define mp make_pair
#define X first
#define Y second

using namespace std;

using ll = long long;
using ull = unsigned long long;
using ld = long double;
mt19937_64 gen(time(nullptr));

struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    // http://xorshift.di.unimi.it/splitmix64.c
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }

  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};

ll mod = 1e9 + 7;

namespace {
ll mul(ll a, ll b) {
  ll val = a * b - (ll)((ld)a * b / mod) * mod;
  if (val < 0)
    val += mod;
  if (val >= mod)
    val -= mod;
  return val;
}
ll poww(ll a, ll b) {
  ll val = 1;
  a %= mod;
  while (b > 0) {
    if (b % 2)
      val = mul(a, val);
    a = mul(a, a);
    b >>= 1;
  }
  return val % mod;
}
ll inv(ll a) { return poww(a, mod - 2); }
} // namespace

ll const maxn = 3e5 + 5;

signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  // freopen("output.txt", "w", stdout);
  ll n;
  cin >> n;
  vector<ll> a(n);
  ;
  for (ll i = 0; i < n; i++)
    cin >> a[i];
  vector<ll> b(n);
  if (n == 2) {
    cout << a[1] << " " << a[0];
    return 0;
  }
  b[0] = 0;
  for (ll i = 0; i < n; i++) {
    ll sm = 0;
    for (ll j = i + 1; j <= i + 3; j++) {
      sm ^= a[j % n];
    }
    b[i] = sm;
    // b[i] = (a[i - 1] ^ a[i]) ^ b[i - 1];
  }
  ll sm = 0;
  for (ll i = 0; i < n; i++)
    sm ^= b[i];
  for (ll i = 0; i < n; i++) {
    assert(a[i] == (sm ^ (b[i])));
  }
  for (ll i = 0; i < n; i++) {
    cout << b[i] << " ";
  }

  return 0;
}

/*

*/