该算法不包含括号运算！
主要思路是利用递归算法
1、先把减法变成加法，把除法变成乘法
2、截取优先级低的算式，将优先级高的算式递归调用本身算出结果

直接上代码

func calcuterResult(input: String) -> Double {
    let newStr = input.replacingOccurrences(of: "-", with: "+-") //减法变加法
        .replacingOccurrences(of: "/", with: "*1/") //除法变乘法
    return calcuter(input: newStr)
}

func calcuter(input: String) -> Double {

    if input.contains("+") { // 加法运算

        var result = 0.0
        let values = input.components(separatedBy: "+") 
        for value in values {

            guard !value.isEmpty else { continue }
            if let num = Double(value) {
                result = result + num
            } else {
                result = result + calcuter(input: value)
            }
        }
        return result

    } else { // 乘法结果

        var result = 1.0
        let values = input.components(separatedBy: "*")
        for value in values {

            guard !value.isEmpty else { continue }
            if let num = Double(value) {
                result = result * num
            } else {
                // "1/2" ,"1/5"
                let coms = value.components(separatedBy: "/")
                if let num = coms.last {
                    result = result * 1 / (Double(num) ?? 1)
                }
            }
        }
        return result
    }
}