题解 | #表达式求值#

exp_list = []
h_exp_list = []
stack_1 = []
stack_2 = []

def strtolist():
    """处理输入"""
    r_exp = list(input())
    for i in range(len(r_exp)):
        if (r_exp[i] == '[') or (r_exp[i] == '{'):
            r_exp[i] = '('
        if (r_exp[i] == ']') or (r_exp[i] == '}'):
            r_exp[i] = ')'

    flag = 1
    number = 0
    

    for i in range(len(r_exp)):
        # 对负数的处理，第一个数是负数和左括号后面有减号就是负数。
        if (i == 0 and r_exp[i] == '-') or (r_exp[i - 1] == '(' and '-' == r_exp[i]):
            flag = -1
        elif '0' <= r_exp[i] <= '9':
            number = number * 10 + int(r_exp[i])
            if len(r_exp) == i+1 or r_exp[i+1] < '0' or r_exp[i+1] > '9' :
                exp_list.append(flag * number)
                number = 0
                flag = 1
        else:
            exp_list.append(r_exp[i])


def process(ch):
    "中缀表达式转后缀表达式"
    if ('(' == ch):           # 读到左括号时，将其压入堆栈中。
        stack_1.append(ch)
    # 遇到右括号时，右括号本身不入栈，从栈顶开始弹出操作符，放入输出流，直到遇到一个左括号为止，将这个左括号弹出，但是不放入输出流。
    elif (')' == ch):
        while True:
            tmp = stack_1.pop()
            if tmp != '(':
                h_exp_list.append(tmp)
            else:
                break
    elif '*' == ch or '/' == ch:
        while True:
            if 0 == len(stack_1):
                stack_1.append(ch)
                break
            else:
                tmp = stack_1.pop()
                if (tmp == '*') or (tmp == '/'):
                    h_exp_list.append(tmp)
                else:
                    stack_1.append(tmp)
                    stack_1.append(ch)
                    break
    elif '+' == ch or '-' == ch:
        while True:
            if 0 == len(stack_1):
                stack_1.append(ch)
                break
            else:
                tmp = stack_1.pop()
                if '(' != tmp:
                    h_exp_list.append(tmp)
                else:
                    stack_1.append(tmp)
                    stack_1.append(ch)
                    break
    else:
        h_exp_list.append(ch)


strtolist()

for ch in exp_list:
    process(ch)

L = len(stack_1)
while L:
    h_exp_list.append(stack_1.pop())
    L -= 1
def calcuValue(h_exp_list):
    for i in range(len(h_exp_list)):
        if h_exp_list[i] in ['+', '-', '*', '/']:
            op2 = stack_2.pop()
            op1 = stack_2.pop()
            if h_exp_list[i] == '+':
                res = op1 + op2
            elif h_exp_list[i] == '-':
                res = op1 - op2
            elif h_exp_list[i] == '*':
                res = op1 * op2
            elif h_exp_list[i] == '/':
                res = op1 / op2
            stack_2.append(res)
        else:
            stack_2.append(h_exp_list[i])

calcuValue(h_exp_list)
result_list = str(stack_2[0]).split('.')
if result_list[-1] == '0':
    print(result_list[0])
else:
    print(stack_2[0])

PS:这个题目的答案是复用的四则运算那道题的。 PPS：关于详细的思路解法，可以看这里：https://blog.csdn.net/zy010101/article/details/82728726