Problem p02651 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
#define ll long long
#define REP(i, a, b) for (ll i = a; i < b; i++)
#define REPI(i, a, b) for (ll i = b - 1; i >= a; i--)
#define i_os                                                                   \\
  ios::sync_with_stdio(0);                                                     \\
  cin.tie(0);                                                                  \\
  cout.tie(0);
#define INF (ll)1e18 + 100
#define endl "\\n"
#define p0(a) cout << a << " "
#define p1(a) cout << a << endl
#define p2(a, b) cout << a << " " << b << endl
#define p3(a, b, c) cout << a << " " << b << " " << c << endl
#define p4(a, b, c, d) cout << a << " " << b << " " << c << " " << d << endl
/*SOME BITMASK KNOWLEDGE
1)x & (x - 1):sets the last one bit of x to zero
power of two exactly when x & (x − 1) = 0.
2)x & -x:sets all the one bits to zero, except last one bit
3)x | (x - 1):inverts all the bits after the last one bit*/
#define o_set                                                                  \\
  tree<int, null_type, less<int>, rb_tree_tag,                                 \\
       tree_order_statistics_node_update>
#define o_setll                                                                \\
  tree<ll, null_type, less<ll>, rb_tree_tag, tree_order_statistics_node_update>
typedef tree<pair<ll, ll>, null_type, less<pair<ll, ll>>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set;
typedef tree<ll, null_type, less<ll>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set1;
// 1. order_of_key(k) : number of elements strictly lesser than k
// 2. find_by_order(k) : k-th element in the set

int main() {
  i_os;
  ll t;
  cin >> t;
  while (t--) {
    ll n;
    cin >> n;
    ll arr[n];
    REP(i, 0, n) { cin >> arr[i]; }
    string s;
    cin >> s;
    vector<ll> st;
    // st.insert(0);
    bool f = 0;

    REPI(i, 0, n) {
      ll v = arr[i];
      for (auto it : st) {
        v = min(v, v ^ it);
      }
      if (v) {
        if (s[i] == '1') {
          cout << 1 << endl;
          f = 1;
          break;
        } else {
          st.push_back(v);
        }
      }
    }
    if (f == 0) {
      cout << 0 << endl;
    }
  }
  return 0;
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
#define ll long long
#define REP(i, a, b) for (ll i = a; i < b; i++)
#define REPI(i, a, b) for (ll i = b - 1; i >= a; i--)
#define i_os                                                                   \\
  ios::sync_with_stdio(0);                                                     \\
  cin.tie(0);                                                                  \\
  cout.tie(0);
#define INF (ll)1e18 + 100
#define endl "\\n"
#define p0(a) cout << a << " "
#define p1(a) cout << a << endl
#define p2(a, b) cout << a << " " << b << endl
#define p3(a, b, c) cout << a << " " << b << " " << c << endl
#define p4(a, b, c, d) cout << a << " " << b << " " << c << " " << d << endl
/*SOME BITMASK KNOWLEDGE
1)x & (x - 1):sets the last one bit of x to zero
power of two exactly when x & (x − 1) = 0.
2)x & -x:sets all the one bits to zero, except last one bit
3)x | (x - 1):inverts all the bits after the last one bit*/
#define o_set                                                                  \\
  tree<int, null_type, less<int>, rb_tree_tag,                                 \\
       tree_order_statistics_node_update>
#define o_setll                                                                \\
  tree<ll, null_type, less<ll>, rb_tree_tag, tree_order_statistics_node_update>
typedef tree<pair<ll, ll>, null_type, less<pair<ll, ll>>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set;
typedef tree<ll, null_type, less<ll>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set1;
// 1. order_of_key(k) : number of elements strictly lesser than k
// 2. find_by_order(k) : k-th element in the set

int main() {
  i_os;
  ll t;
  cin >> t;
  while (t--) {
    ll n;
    cin >> n;
    ll arr[n];
    REP(i, 0, n) { cin >> arr[i]; }
    string s;
    cin >> s;
    set<ll> st;
    st.insert(0);
    bool f = 0;

    REPI(i, 0, n) {
      if (s[i] == '1') {
        if (st.find(arr[i]) != st.end()) {
          continue;
        } else {
          cout << 1 << endl;
          f = 1;
          break;
        }
      } else {
        set<ll> s1;
        for (auto it : st) {
          s1.insert(it ^ arr[i]);
        }
        for (auto it : s1) {
          st.insert(it);
        }
      }
    }
    if (f == 0) {
      cout << 0 << endl;
    }
  }
  return 0;
}