Sum of All Subset XOR Totals

给一个数组, 求其中subset的xor的和.

class Solution {
public:
    int sum = 0;
    int subsetXORSum(vector<int>& nums) {
        sub(nums, 0, 0);
        return sum;
    }
    void sub(vector<int>& nums, int pos, int n){
        sum += n;
        for(int i = pos; i < nums.size(); i++) {
            n ^= nums[i];
            sub(nums, i + 1, n); 
            n ^= nums[i];
        }
    }
};