// Subsets — MEDIUM
// Category: backtracking
Given an integer array `nums` of **unique** elements, return all possible subsets (the power set).
The solution set **must not** contain duplicate subsets. Return the solution in **any order**.
Example: nums = [1,2,3]
Output: [[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3]]