Subsets - Include or Exclude

For each element, you have two choices: include it or exclude it. This creates a binary decision tree with $2^n$ leaves (one for each subset).

You can use the same backtracking pattern from combinations. At each recursive call, you add the current combination (no matter the size) to results, then try adding each remaining number.

Unlike combinations where you wait until size equals $k$, here you record every state. The empty subset happens when you start (before adding any elements).