Combination Sum

Word Search

Permutations

Find target-sum combinations when candidate values may be reused.

Return unique combinations summing to `target`. Candidates are distinct, may be reused, and output follows input-choice DFS order.

## Example
```json
Input: {"candidates":[2,3,6,7],"target":7}
Output: [[2,2,3],[7]]
```

## Constraints
- 1 <= candidates.length <= 30
- 2 <= candidates[i] <= 40
- 1 <= target <= 40

## Follow-up
How does sorting enable early pruning?

The key pattern is: Backtracking branches between reusing and skipping each candidate. Identify the state and invariant before coding.

Recognize the Pattern

Advance one state transition at a time. Mark or update state before exploring dependent work.

Build the Algorithm

Test empty or minimal input, skewed shapes, duplicates where allowed, and impossible outcomes.