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Combination Sum at Microsoft

The problem at Microsoft

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

Microsoft uses this problem to assess growth mindset, collaborative, product-minded. Expect Microsoft-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 Microsoft looks for

Growth mindset — learn from feedback and failure.
Collaboration — how you work across teams.
Customer and product focus — especially for product teams.
Solid CS fundamentals in coding and design.

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