Note:

1. Very smart way of calculating how many difference from bits: https://en.wikipedia.org/wiki/Hamming_distance#Algorithm_example

2. Another way is counting how many 1s and 0s per bits. Then the contribution of that bit will bt C(1, k) * C(1, n - k).

class Solution {    public int totalHammingDistance(int[] nums) {        if (nums.length < 2) {            return 0;        }                int result = 0;;        for (int i = 0; i < nums.length; i++) {            for (int j = i + 1; j < nums.length; j++) {                result += getDistance(nums[i], nums[j]);            }        }        return result;    }        private int getDistance(int a, int b) {        int c = a ^ b;        int result = 0;        while (c != 0) {            result++;            c &= c - 1;        }        return result;    }}

class Solution {    public int totalHammingDistance(int[] nums) {        if (nums.length < 2) {            return 0;        }                int result = 0;;        for (int i = 0; i < 32; i++) {            int ones = 0;            for (int j = 0; j < nums.length; j++) {                ones += (nums[j] >> i) & 1;            }            result += ones * (nums.length - ones);        }        return result;    }}