题解 | #表达式求值#

import math

ops = {'+':1, '-':1,'*':2,'/':2 }

def cal(a, b, op): # 计算两个数的结果
    if op == "+":
        return a + b
    elif op == "-":
        return a - b
    elif op == "*":
        return a * b
    elif op == "/":
        return math.floor(a / b)


def handleStack(stack_num, stack_op): # 从栈中拿出数字和运算符进行计算，并将结果压栈
    b = stack_num.pop()
    a = stack_num.pop()
    cop = stack_op.pop()
    stack_num.append(cal(a, b, cop))


def comp(op1, op2): # 比较运算符的优先级
    if op1 == "(":
        return False
    else:
        return ops[op1] >= ops[op2]

while True:
    try:
        s = input()
        num = ''
        stack_num = []
        stack_op = []
        i = 0
        while i < len(s):
            # 遇到数字或者负号
            while (i < len(s) 
                and (s[i].isdigit()
                # 0位置的减号，或者左括号之后的减号，是负号
                or (s[i] == '-' and (i == 0 or s[i-1] == '(')))):
                num += s[i]
                i += 1 # 一直遍历到不是数字的位置
            if num != '':
                stack_num.append(int(num))
                num = ''
            if i == len(s):
                break

            if s[i] in ops: # 如果是运算符，需要判断运算符优先级，优先级更高的出栈并计算
                while (len(stack_op) > 0 and comp(stack_op[-1], s[i])):
                    handleStack(stack_num, stack_op)
                stack_op.append(s[i])
            elif s[i] == '(': # 如果是左括号，入栈
                stack_op.append(s[i])
            elif s[i] == ')': # 如果是右括号，一直计算直到左括号出栈
                while stack_op[-1] != '(':
                    handleStack(stack_num, stack_op)
                stack_op.pop()
            i += 1

        while(len(stack_op)):  # 如果还有运算符，继续出栈计算
            handleStack(stack_num, stack_op)
        print(stack_num[0])
    except:
        break