strAddition.swift

func strToNum(str: String) -> UInt32 {
        // ascii helper -> "0" = 48 && "9" = 57 | "+" = 43
        let arr = str.map { String($0) }
        var num:UInt32 = 0
        for item in arr  {
            let asciiVal = UnicodeScalar(item)!.value
            if(asciiVal < 48 || asciiVal > 57) {
                continue
            }
            num = (num*10)+(asciiVal-48)
        }
        return num
}

func strAddition(str1:String, str2:String) -> String {
    // assuming + sign does not work but += sign does work!!!
    var res:UInt32 = strToNum(str: str1)
    res += strToNum(str: str2)
    return String(res)
}

print(strAddition(str1:"3", str2: "3"))