1322B - Present - CodeForces Solution

#include <bits/stdc++.h>
#define int long long
using namespace std;

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);

    int n;
    cin >> n;

    vector<int> a(n);
    for(int i = 0; i < n;i++) cin >> a[i];

    int ans = 0;

    for(int i = 40; i >= 0;i--) {
        for(auto &e: a) e &= (2ll << i) - 1;
        sort(a.begin(), a.end());

        int cnt = 0;
        for(int j = 0; j < n;j++) {
            cnt += a.end() - lower_bound(a.begin(), a.end(), (1ll << i) - a[j]);
            cnt -= a.end() - lower_bound(a.begin(), a.end(), (2ll << i) - a[j]);
            cnt += a.end() - lower_bound(a.begin(), a.end(), (3ll << i) - a[j]);
            if(2*a[j] >= (1ll << i)) cnt--;
            if(2*a[j] >= (2ll << i)) cnt++;
            if(2*a[j] >= (3ll << i)) cnt--;
        }
        cnt /= 2;

        ans += (1ll << i) * (cnt % 2);
    }

    cout << ans << "\n";
}