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：取出兩個數(shù)字計算值，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)
 // 遍歷整個表達(dá)式
 for i := 0; i < expLen; i++ {
  char := string(exp[i])
  switch char {
  case " ":
continue
  case "(":
// 左括號直接入棧
stack.Push("(")
  case ")":
// 右括號則彈出元素直到遇到左括號
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)先級的運(yùn)算符，不斷彈出，直到遇見更低優(yōu)先級運(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
}