func calculate(postfix string) int {
    stack := stack.ItemStack{}
    fixLen := len(postfix)
    for i := 0; i < fixLen; i++ {
        nextChar := string(postfix[i])
        // 數(shù)字：直接壓棧
        if unicode.IsDigit(rune(postfix[i])) {
            stack.Push(nextChar)
        } else {
            // 操作符：取出兩個(gè)數(shù)字計(jì)算值，再將結(jié)果壓棧
            num1, _ := strconv.Atoi(stack.Pop())
            num2, _ := strconv.Atoi(stack.Pop())
            switch nextChar {
            case "+":
                stack.Push(strconv.Itoa(num1 + num2))
            case "-":
                stack.Push(strconv.Itoa(num1 - num2))
            case "*":
                stack.Push(strconv.Itoa(num1 * num2))
            case "/":
                stack.Push(strconv.Itoa(num1 / num2))
            }
        }
    }
    result, _ := strconv.Atoi(stack.Top())
    return result
}

// 中綴表達(dá)式轉(zhuǎn)后綴表達(dá)式
func infix2ToPostfix(exp string) string {
    stack := stack.ItemStack{}
    postfix := ""
    expLen := len(exp)
    // 遍歷整個(gè)表達(dá)式
    for i := 0; i < expLen; i++ {
        char := string(exp[i])
        switch char {
        case " ":
            continue
        case "(":
            // 左括號(hào)直接入棧
            stack.Push("(")
        case ")":
            // 右括號(hào)則彈出元素直到遇到左括號(hào)
            for !stack.IsEmpty() {
                preChar := stack.Top()
                if preChar == "(" {
                    stack.Pop() // 彈出 "("
                    break
                }
                postfix += preChar
                stack.Pop()
            }
            // 數(shù)字則直接輸出
        case "0", "1", "2", "3", "4", "5", "6", "7", "8", "9":
            j := i
            digit := ""
            for ; j < expLen && unicode.IsDigit(rune(exp[j])); j++ {
                digit += string(exp[j])
            }
            postfix += digit
            i = j - 1 // i 向前跨越一個(gè)整數(shù)，由于執(zhí)行了一步多余的 j++，需要減 1
        default:
            // 操作符：遇到高優(yōu)先級(jí)的運(yùn)算符，不斷彈出，直到遇見(jiàn)更低優(yōu)先級(jí)運(yùn)算符
            for !stack.IsEmpty() {
                top := stack.Top()
                if top == "(" || isLower(top, char) {
                    break
                }
                postfix += top
                stack.Pop()
            }
            // 低優(yōu)先級(jí)的運(yùn)算符入棧
            stack.Push(char)
        }
    }
    // 棧不空則全部輸出
    for !stack.IsEmpty() {
        postfix += stack.Pop()
    }
    return postfix
}
// 比較運(yùn)算符棧棧頂 top 和新運(yùn)算符 newTop 的優(yōu)先級(jí)高低
func isLower(top string, newTop string) bool {
    // 注意 a + b + c 的后綴表達(dá)式是 ab + c +，不是 abc + +
    switch top {
    case "+", "-":
        if newTop == "*" || newTop == "/" {
            return true
        }
    case "(":
        return true
    }
    return false
}