Sum of pairwise Hamming Distance

• Time : $O(nlog(A_i))$
• Space : $O(1)$

int Solution::hammingDistance(const vector<int> &A) {
    long long n = A.size(), res = 0, mod = 1e9 + 7;
    vector<int> freq(32);
    for(auto a : A) {
        for(int i = 0; i < 32; i++) {
            if(a & (1<<i)) freq[i]++;
        }
    }
    for(auto a : A) {
        for(int i = 0; i < 32; i++) {
            if(a & (1<<i)) res = (res + n - freq[i]) % mod;
            else res = (res + freq[i]) % mod;
        }
    }
    return res;
}