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func constructArr(_ a: [Int]) -> [Int] {
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    guard a.isEmpty == false else {
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        return []
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    }
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    var res: [Int] = []
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    /// 某位之前的乘积
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    var dpL: [Int : Int] = [0 : 1]
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    /// 某位之后的乘积
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    var dpR: [Int : Int] = [(a.count - 1) : 1]
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    for idx in 1..<a.count {
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        let lastIdx = idx - 1
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        if let last = dpL[lastIdx] {
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            dpL[idx] = last * a[lastIdx]
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        }
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        let backwordIdx = a.count - idx - 1
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        if let last = dpR[backwordIdx + 1] {
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            dpR[backwordIdx] = last * a[backwordIdx + 1]
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        }
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    }
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    for idx in 0..<a.count {
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        if let l = dpL[idx], let r = dpR[idx] {
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            res.append(l * r)
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        }
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    }
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    return res
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}