Problem p02626 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <algorithm>
#include <bitset>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <unordered_map>
#include <vector>
// #include <bits/stdc++.h>
#define ALL(x) (x).begin(), (x).end()
#define sz(a) int(a.size())
#define rep(i, x, n) for (int i = x; i < n; i++)
#define repd(i, x, n) for (int i = x; i <= n; i++)
#define pii pair<int, int>
#define pll pair<long long, long long>
#define gbtb ios::sync_with_stdio(false), cin.tie(0), cout.tie(0)
#define MS0(X) memset((X), 0, sizeof((X)))
#define MSC0(X) memset((X), '\\0', sizeof((X)))
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define eps 1e-6
#define chu(x)                                                                 \\
  if (DEBUG_Switch)                                                            \\
  cout << "[" << #x << " " << (x) << "]" << endl
#define du3(a, b, c) scanf("%d %d %d", &(a), &(b), &(c))
#define du2(a, b) scanf("%d %d", &(a), &(b))
#define du1(a) scanf("%d", &(a));
using namespace std;
typedef long long ll;
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
ll powmod(ll a, ll b, ll MOD) {
  if (a == 0ll) {
    return 0ll;
  }
  a %= MOD;
  ll ans = 1;
  while (b) {
    if (b & 1) {
      ans = ans * a % MOD;
    }
    a = a * a % MOD;
    b >>= 1;
  }
  return ans;
}
ll poww(ll a, ll b) {
  if (a == 0ll) {
    return 0ll;
  }
  ll ans = 1;
  while (b) {
    if (b & 1) {
      ans = ans * a;
    }
    a = a * a;
    b >>= 1;
  }
  return ans;
}
void Pv(const vector<int> &V) {
  int Len = sz(V);
  for (int i = 0; i < Len; ++i) {
    printf("%d", V[i]);
    if (i != Len - 1) {
      printf(" ");
    } else {
      printf("\\n");
    }
  }
}
void Pvl(const vector<ll> &V) {
  int Len = sz(V);
  for (int i = 0; i < Len; ++i) {
    printf("%lld", V[i]);
    if (i != Len - 1) {
      printf(" ");
    } else {
      printf("\\n");
    }
  }
}
inline long long readll() {
  long long tmp = 0, fh = 1;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-')
      fh = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9')
    tmp = tmp * 10 + c - 48, c = getchar();
  return tmp * fh;
}
inline int readint() {
  int tmp = 0, fh = 1;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-')
      fh = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9')
    tmp = tmp * 10 + c - 48, c = getchar();
  return tmp * fh;
}
void pvarr_int(int *arr, int n, int strat = 1) {
  if (strat == 0) {
    n--;
  }
  repd(i, strat, n) { printf("%d%c", arr[i], i == n ? '\\n' : ' '); }
}
void pvarr_LL(ll *arr, int n, int strat = 1) {
  if (strat == 0) {
    n--;
  }
  repd(i, strat, n) { printf("%lld%c", arr[i], i == n ? '\\n' : ' '); }
}
const int maxn = 1000010;
const int inf = 0x3f3f3f3f;
/*** TEMPLATE CODE * * STARTS HERE ***/
#define DEBUG_Switch 0
int n;
ll a[maxn];
int main() {
#if DEBUG_Switch
  freopen("C:\\\\code\\\\input.txt", "r", stdin);
#endif
  // freopen("C:\\\\code\\\\output.txt","r",stdin);
  cin >> n;
  repd(i, 1, n) { cin >> a[i]; }
  ll x = 0ll;
  repd(i, 3, n) { x ^= a[i]; }
  ll s = a[1] + a[2];
  // a+b = s
  // a^b = x
  //
  // a+b=(a^b)+2*(a&b)
  // s=x+2*(a&b)
  //
  // (s-x)/2=a&b=d
  // a^=x
  //

  ll d = s - x;
  // cout << d << endl;
  if (d < 0 || (d & 1)) {
    // cout << "first" << endl;
    cout << -1 << endl;
  } else {
    d /= 2;
    if (d > a[1] || (d & x) > 0) {
      // cout << "second" << endl;
      cout << -1 << endl;
    } else {
      ll aa = d;
      for (int i = 60; i >= 0; --i) {
        if (x & (1ll << i)) {
          if (aa + (1ll << i) <= a[1]) {
            aa += (1ll << i);
          }
        }
      }
      if (aa == 0) {
        cout << -1 << endl;
      } else {
        cout << a[1] - aa << endl;
      }
    }
  }

  return 0;
}


Error: TLE

Fixed submission:

#include <algorithm>
#include <bitset>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <unordered_map>
#include <vector>
// #include <bits/stdc++.h>
#define ALL(x) (x).begin(), (x).end()
#define sz(a) int(a.size())
#define rep(i, x, n) for (int i = x; i < n; i++)
#define repd(i, x, n) for (int i = x; i <= n; i++)
#define pii pair<int, int>
#define pll pair<long long, long long>
#define gbtb ios::sync_with_stdio(false), cin.tie(0), cout.tie(0)
#define MS0(X) memset((X), 0, sizeof((X)))
#define MSC0(X) memset((X), '\\0', sizeof((X)))
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define eps 1e-6
#define chu(x)                                                                 \\
  if (DEBUG_Switch)                                                            \\
  cout << "[" << #x << " " << (x) << "]" << endl
#define du3(a, b, c) scanf("%d %d %d", &(a), &(b), &(c))
#define du2(a, b) scanf("%d %d", &(a), &(b))
#define du1(a) scanf("%d", &(a));
using namespace std;
typedef long long ll;
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
ll powmod(ll a, ll b, ll MOD) {
  if (a == 0ll) {
    return 0ll;
  }
  a %= MOD;
  ll ans = 1;
  while (b) {
    if (b & 1) {
      ans = ans * a % MOD;
    }
    a = a * a % MOD;
    b >>= 1;
  }
  return ans;
}
ll poww(ll a, ll b) {
  if (a == 0ll) {
    return 0ll;
  }
  ll ans = 1;
  while (b) {
    if (b & 1) {
      ans = ans * a;
    }
    a = a * a;
    b >>= 1;
  }
  return ans;
}
void Pv(const vector<int> &V) {
  int Len = sz(V);
  for (int i = 0; i < Len; ++i) {
    printf("%d", V[i]);
    if (i != Len - 1) {
      printf(" ");
    } else {
      printf("\\n");
    }
  }
}
void Pvl(const vector<ll> &V) {
  int Len = sz(V);
  for (int i = 0; i < Len; ++i) {
    printf("%lld", V[i]);
    if (i != Len - 1) {
      printf(" ");
    } else {
      printf("\\n");
    }
  }
}
inline long long readll() {
  long long tmp = 0, fh = 1;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-')
      fh = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9')
    tmp = tmp * 10 + c - 48, c = getchar();
  return tmp * fh;
}
inline int readint() {
  int tmp = 0, fh = 1;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-')
      fh = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9')
    tmp = tmp * 10 + c - 48, c = getchar();
  return tmp * fh;
}
void pvarr_int(int *arr, int n, int strat = 1) {
  if (strat == 0) {
    n--;
  }
  repd(i, strat, n) { printf("%d%c", arr[i], i == n ? '\\n' : ' '); }
}
void pvarr_LL(ll *arr, int n, int strat = 1) {
  if (strat == 0) {
    n--;
  }
  repd(i, strat, n) { printf("%lld%c", arr[i], i == n ? '\\n' : ' '); }
}
const int maxn = 1000010;
const int inf = 0x3f3f3f3f;
/*** TEMPLATE CODE * * STARTS HERE ***/
#define DEBUG_Switch 0
int n;
ll a[maxn];
int main() {
#if DEBUG_Switch
  freopen("C:\\\\code\\\\input.txt", "r", stdin);
#endif
  // freopen("C:\\\\code\\\\output.txt","r",stdin);
  cin >> n;
  repd(i, 1, n) { cin >> a[i]; }
  ll x = 0ll;
  repd(i, 3, n) { x ^= a[i]; }

  int isok = 0;
  repd(i, 0, a[1] - 1) {
    ll num1 = a[1] - i;
    ll num2 = a[2] + i;
    if ((num2 ^ num1 ^ x) == 0) {
      cout << i << endl;
      return 0;
    }
  }
  cout << -1 << endl;
  return 0;
}