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Split the array into two halves. Generate all 220 subset sums for each half.
1. Compute all sums for the first half: O(2n/2).
2. Compute all sums for the second half: O(2n/2).
3. Sort one list.
4. For each sum s1 in the first list, binary search for goal−s1 in the second list to find the closest match.
Total time: O(2n/2⋅n).
