XOR-ing the Subarrays!

• Time : $O(n)$
• Space : $O(1)$

int Solution::solve(vector<int> &A) {
    int res = 0, n = A.size();
    for(int i = 0; i < n; i++) {
        int req = (i + 1) * (n - i);
        if(req & 1) res ^= A[i];
    }
    return res;
}