Subsets - Include/Exclude Pattern

For each element, you have two choices: include it or exclude it. This naturally forms a binary decision tree.

$1.$ Start with an empty current subset.

$2.$ At each position, either include nums[i] and recurse, or skip it and recurse.

$3.$ When you've processed all elements, add current subset to results.

The number of subsets is $2^n$.