Subsets

Invert Binary Tree

Top K Frequent Elements

Generate every subset in the power set of a distinct integer array.

Given an integer array `nums` of unique values, return every subset. Preserve input order inside subsets and use include-first depth-first output order.

## Example
```
Input: {"nums":[1,2]}
Output: [[1,2],[1],[2],[]]
```

**Constraints**
- 0 <= nums.length <= 10
- All values are unique.

**Follow-up**
How can bit masks enumerate the same power set?

- Identify the required output and the state that changes.
- Use the examples to rule out tempting but incorrect interpretations.

Understand the Goal

- This problem is best exposed by Backtracking makes an include/exclude decision for each value.
- Maintain only the state needed for the next operation.

Choose the Core Pattern

- Process each state once when possible.
- Record state before scheduling dependent work to avoid duplicates.

Build the Algorithm

- Verify the smallest input, impossible cases, and repeated values where allowed.
- Keep output ordering deterministic.

Check Edge Cases

- Time complexity is O(n*2^n).
- Space complexity is O(n*2^n).