Problem p02626 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
using namespace std;
template <typename A, typename B> string to_string(pair<A, B> p);
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
string to_string(const string &s) { return '"' + s + '"'; }
string to_string(const char *s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template <size_t N> string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
void debug_out() { cout << endl; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cout << " " << to_string(H);
  debug_out(T...);
}
#ifdef LOCAL
#define dbg(...) cout << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define dbg(...) 42
#endif
const long long mod = 1e9 + 7; // CHECK
long long power(long long a, long long b, long long m = mod) {
  if (b < 0) {
    return power(power(a, -b), mod - 2);
  }
  long long x = 1;
  while (b) {
    if (b & 1) {
      x = 1ll * x * a % m;
    }
    a = 1ll * a * a % m;
    b /= 2;
  }
  return x;
}
#define ll long long
const int INT_INF = 1e9 + 9;
const ll LL_INF = 2e18;
const int N = 3e5 + 9; // check N, mod

signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  vector<ll> a(n);
  ll xr = 0;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    xr ^= a[i];
  }
  if (xr == 0) {
    cout << "0\\n";
    return 0;
  }
  xr = xr ^ a[0] ^ a[1];
  // a & b = (a[0] + a[1] - xr) / 2
  // a ^ b = xr
  // we want a to be as big as possible <= a[0]
  if ((a[0] + a[1] - xr) & 1) {
    cout << "-1\\n";
    return 0;
  }
  ll ad = (a[0] + a[1] - xr) / 2;
  const int B = 41;
  ll b = ad, c = ad;
  for (int bit = B - 1; bit >= 0; bit--) {
    // 1 0 = 1 0
    // 0 0 = 0 0
    // 1 1 = 0 1
    int u = (xr >> bit & 1);
    int v = (ad >> bit & 1);
    if (u == 1 && v == 1) {
      cout << "-1\\n";
      return 0;
    }
    if (u == 0 && v == 0) {

    } else if (u == 0 && v == 1) {

    } else if (u == 1 && v == 0) {
      if ((b | (1ll << bit)) > a[0]) {
        c |= (1ll << bit);
      } else {
        b |= (1ll << bit);
      }
    }
  }
  if (b == 0 || b > a[0]) {
    cout << "-1\\n";
    return 0;
  }
  // dbg(b, c, xr, ad);
  cout << (a[0] - b) << '\\n';

  return 0;
}


Error: 0

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
template <typename A, typename B> string to_string(pair<A, B> p);
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
string to_string(const string &s) { return '"' + s + '"'; }
string to_string(const char *s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template <size_t N> string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
void debug_out() { cout << endl; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cout << " " << to_string(H);
  debug_out(T...);
}
#ifdef LOCAL
#define dbg(...) cout << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define dbg(...) 42
#endif
const long long mod = 1e9 + 7; // CHECK
long long power(long long a, long long b, long long m = mod) {
  if (b < 0) {
    return power(power(a, -b), mod - 2);
  }
  long long x = 1;
  while (b) {
    if (b & 1) {
      x = 1ll * x * a % m;
    }
    a = 1ll * a * a % m;
    b /= 2;
  }
  return x;
}
#define ll long long
const int INT_INF = 1e9 + 9;
const ll LL_INF = 2e18;
const int N = 3e5 + 9; // check N, mod
const int M = 2e6 + 9;
long long fact[M], ifact[M], inv[M];
long long ncr(long long n, long long r) {
  assert(n < M);
  if (r < 0 || n < 0 || r > n)
    return 0;
  long long ret = fact[n] * ifact[n - r] % mod * ifact[r] % mod;
  return ret;
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  vector<ll> a(n);
  ll xr = 0;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    xr ^= a[i];
  }
  if (xr == 0) {
    cout << "0\\n";
    return 0;
  }
  xr = xr ^ a[0] ^ a[1];
  // a & b = (a[0] + a[1] - xr) / 2
  // a ^ b = xr
  // we want a to be as big as possible <= a[0]
  if ((a[0] + a[1] - xr) & 1) {
    cout << "-1\\n";
    return 0;
  }
  ll ad = (a[0] + a[1] - xr) / 2;
  const int B = 40;
  ll b = ad, c = ad;
  for (int bit = B - 1; bit >= 0; bit--) {
    // 1 0 = 1 0
    // 0 0 = 0 0
    // 1 1 = 0 1
    int u = (xr >> bit & 1);
    int v = (ad >> bit & 1);
    if (u == 1 && v == 1) {
      cout << "-1\\n";
      return 0;
    }
    if (u == 0 && v == 0) {

    } else if (u == 0 && v == 1) {

    } else if (u == 1 && v == 0) {
      if ((b | (1ll << bit)) > a[0]) {
        c |= (1ll << bit);
      } else {
        b |= (1ll << bit);
      }
    }
  }
  // dbg(b, c);
  assert(c >= a[1]);
  if (b == 0) {
    cout << "-1\\n";
    return 0;
  }
  // dbg(b, c, xr, ad);
  cout << (a[0] - b) << '\\n';

  return 0;
}