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Combination Sum at Apple — Interview Guide

The problem at Apple

Given distinct candidates and a target, return all unique combinations that sum to the target. Each candidate may be used unlimited times.

Apple uses this problem to assess craft, detail, and domain depth. Expect Apple-specific follow-ups that tie the solution back to production reasoning and the company's engineering bar.

Key concepts

Sort candidates first so you can prune when the running sum exceeds the target.
Recurse with the same index (unlimited use) but never go back to an earlier index (avoids permutations of the same combo).
Copy the current combination before adding it to the result.

Approach

def combination_sum(cands, target):
    cands.sort()
    res = []
    def bt(start, path, total):
        if total == target:
            res.append(list(path)); return
        for i in range(start, len(cands)):
            if total + cands[i] > target: break
            path.append(cands[i])
            bt(i, path, total + cands[i])
            path.pop()
    bt(0, [], 0)
    return res

What Apple looks for

Domain expertise relevant to the team.
Craft — performance, edge cases, and polish.
Passion for the product and user experience.
Independence and ownership of a feature end-to-end.

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