简单扩展了下 String 支持转换四则运算的计算字符，若要加入更多支持，可以自己加入扩展。下面的扩展只是把字面上的字符串，简单转化成中缀表达式并且分割成了数组，方便转后缀表达式计算结果。
var str = "{1+2-[(1+2)x2]x2}"
str.ConverCalculateChar()
//str =["(","1","+","2","-","(","(","1","+","2",")","*","2",")","*","2",")"]
extension String {
mutating func ConverCalculateChar() -> [String] {
var console = true //控制合成连续数字字符
var SerialNumber = ""
var strArr = [String]()
self += "A" //临时帮助下循环正常进行
for char in self.characters {
switch char {
case "0","1","2","3","4","5","6","7","8","9",".":
console == true ? (SerialNumber += String(char)) : (SerialNumber = String(char))
console = true
default:
console = false
if !SerialNumber.isEmpty {
strArr.append(SerialNumber)
SerialNumber = ""
}
strArr.append(String(char))
}
}
strArr.removeLast() //删除刚刚的重要临时工"A"
var index = 0
for str in strArr {
switch str { //设定一定的容错性转换
case "X", "x", "×":
strArr[index] = "*"
case "÷":
strArr[index] = "/"
case "（", "{", "[":
strArr[index] = "("
case "）", "}", "]":
strArr[index] = ")"
default:
break
}
index += 1
}
return strArr
}
}
