Amazon asks this in interviews. Your interviewer expects both the brute force explanation and the optimized backtracking. Here's the pattern: when elements can be reused, recurse with start = i instead of i+. When elements can't be reused, recurse with i+. Related: Combination Sum II (no reuse, with duplicates), Combination Sum III (fixed count), Combination Sum IV (count, not list).
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$ curl repovive.com/roadmaps/maang-interview-prep/backtracking/combination-sum-why-this-problem
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