Different Bits Sum Pairwise

• Time : $O(n)$
• Space : $O(1)$

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