1.3 调车场问题

面试题 16.26. 计算器

def calculate(self, s: str) -> int:
        stack,base = [],'0123456789'
        i,n = 0,len(s)
        while i < n:
            if s[i] == " ": 
                i += 1
                continue
            c = s[i] # 保存操作符
            if c in '+-*/': #找到数字的位置
                i += 1  
                while i < n and s[i] == " ":
                    i += 1
            num = 0
            while i < n and s[i] in base: #如果是连续数字，需要更新栈顶数据
                num = num * 10 + int(s[i])
                i += 1
            if c == '-':  # 根据之前保存的操作符对应计算
                num = -num
            elif c == '*':
                num = stack.pop()*num
            elif c == '/':
                tmp = stack.pop()
                num = int(operator.truediv(tmp, num)) # 等同于下面代码 改变python原有除法取整
                # if (tmp > 0 and num < 0):
                #     num = -(tmp // (-num))
                # elif (tmp < 0 and num > 0):
                #     num = -((-tmp) // num)
                # elif (tmp<0 and num<0):
                #     num = (-tmp) // (-num)
                # else:
                #     num = tmp//num
            stack.append(num)
        return sum(stack)