题解 | 表达式求值

#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 返回表达式的值
# @param s string字符串 待计算的表达式
# @return int整型
#
import re
class Solution:
    def solve(self , s: str) -> int:
        # write code here
        stack = []
        ls = self.infix_to_postfix(s)
        for it in ls:
            if it.isdigit():
                stack.append(int(it))
            elif it=="+":
                stack.append(stack.pop()+stack.pop())
            elif it=="*":
                stack.append(stack.pop()*stack.pop())
            elif it=="-":
                stack.append(-(stack.pop()-stack.pop()))
            else:
                pass
        return stack[0]

    def infix_to_postfix(self, s: str) -> list:
        prec = {'+': 1, '-': 1, '*': 2}
        res = []
        stack = []
        tokens = re.findall(r'\d+|[()+*\-]', s)

        for token in tokens:
            if token.isdigit():
                res.append(token)
            elif token == '(':
                stack.append(token)
            elif token == ')':
                while stack and stack[-1] != '(':
                    res.append(stack.pop())
                stack.pop()
            else:
                while stack and stack[-1] != '(' and prec[stack[-1]] >= prec[token]:
                    res.append(stack.pop())
                stack.append(token)

        while stack:
            res.append(stack.pop())

        print(res)
        return res