Count the number of set bits in `num2` (let's call this `k`). This will be the target number of set bits for `x`.

Generate all possible numbers with exactly `k` set bits, then compute `x XOR num1` for each, and return the smallest `x` that minimizes this XOR. (Note: This brute-force approach is inefficient for large numbers.)

Instead of generating all candidates, observe that the optimal `x` will have set bits in positions where `num1` has unset bits (to minimize XOR). Construct `x` by greedily setting the rightmost unset bits in `num1` until `x` has exactly `k` set bits.