Backtracking — Combination Sum

candidates holds distinct positive integers. Return every combination of
candidates that adds up to target. The **same candidate may be used any
number of times**; two combinations are the same when they use the same numbers
the same number of times.

Same template as subsets, with two changes: recurse with i (not i + 1) so
a candidate can be picked again, and prune. Once the running sum would pass
target, no larger candidate can help either, so a sorted input lets you stop
the loop early.

Output order (deterministic)

• Each combination is in ascending order.
• Combinations are listed in lexicographic order.

Examples

solve([2, 3, 6, 7], 7)  → [[2, 2, 3], [7]]
solve([2, 3, 5], 8)     → [[2, 2, 2, 2], [2, 3, 3], [3, 5]]
solve([2], 1)           → []

Constraints

• 1 <= candidates.length <= 30, 2 <= candidates[i] <= 40
• 1 <= target <= 40