Combination Sum - Reuse Strategy

The main insight: when you pick a number, you can pick it again. So when you recurse, you pass the same start index instead of incrementing it.

For example, if you pick $2$ from $[2, 3, 6, 7]$, your next call still starts at index $0$ (where $2$ is). This lets you pick $2$ again. Once you decide to skip $2$, you move to index $1$ and try $3$.

Base case: when your sum equals the target, add the current combination to results. If sum exceeds target, stop exploring that path.