题解 | #表达式求值#

from sys import base_exec_prefix
#
# 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
#
class Solution:
    def solve(self , s ):
        s = s.replace(" ","")
        stack = []
        res = 0
        num = 0
        sign = '+' 
        index = 0
        while index < len(s):
            # 遇到左括号
            if s[index] == '(': 
                bracket_end = index + 1
                # lens是左括号数量，遇到右括号-1
                lens = 1
                
                # bracket_end停在右括号的下一个位置
                while lens > 0: 
                    if s[bracket_end] == '(':
                        lens += 1
                    if s[bracket_end] == ')':
                        lens -= 1
                    bracket_end += 1
                    
                #将括号内的内容视为子问题进入递归
                num = self.solve(s[index + 1: bracket_end - 1]) 
                # index 跳到右括号的位置
                index = bracket_end - 1
                continue
                
            # 遇到字符数字, 存到变量num
            if '0' <= s[index] <= '9':
                num = num * 10 + int(s[index])
                
            # 遇到操作符，将num按规则入栈
            # 遇到最后一个数字， 也将num按规则入栈
            if not '0' <= s[index] <= '9' or index == len(s) - 1:
                #加
                if sign == '+': 
                    stack.append(num)
                #减，加相反数
                elif sign == '-': 
                    stack.append(-1 * num)
                #乘优先计算
                elif sign == '*': 
                    stack.append(stack.pop() * num) 
                sign = s[index]
                num = 0
                
            index = index + 1
            
        #栈中元素相加
        while stack: 
            res += stack.pop()
        return res