Sum of bit differences

Given an integer array of N integers, find sum of bit differences in all pairs that can be formed from array elements. Bit difference of a pair (x, y) is count of different bits at same positions in binary representations of x and y.
For example, bit difference for 2 and 7 is 2. Binary representation of 2 is 010 and 7 is 111 (first and last bits differ in two numbers).

Note: (x, y) and (y, x) are considered two separate pairs.

• Time : O(n)
• Space : O(1)

class Solution{
public:
    long long sumBitDifferences(int arr[], int n) {
        vector<int> bit(32, 0);
        vector<int> nbit(32, 0);
        long long res = 0;
        for(int i = 0; i < n; i++) {
            long long sum = 0;
            for(int j = 0; j < 32; j++) {
                if(arr[i] & (1<<j)) {
                    bit[j]++;
                    res += nbit[j];
                } else {
                    nbit[j]++;
                    res += bit[j];
                }
            }

        }
        return res * 2;
    }
};