Given an integer array of unique elements, return all possible subsets (the power set).
With [1, 2, 3], return:
[[], [1], [2], [3], [1,2], [1,3], [2,3], [1,2,3]].
The solution set must not contain duplicate subsets. Return them in any order.
A subset can have 0 to elements. The empty set and the full array are both valid subsets.
Constraints: length . All elements unique.