2859. Sum of Values at Indices With K Set Bits

You are given a 0-indexed integer array nums and an integer k.

Return an integer that denotes the sum of elements in nums whose corresponding indices have exactly k set bits in their binary representation.

The set bits in an integer are the 1‘s present when it is written in binary.

• For example, the binary representation of 21 is 10101, which has 3 set bits.

class Solution {
public:
    int sumIndicesWithKSetBits(vector<int>& nums, int k) {
        int res = 0;
        for(int i = 0; i < nums.size(); i++) {
            if(__builtin_popcount(i) == k) res += nums[i];
        }
        return res;
    }
};