一眼看过去很简单，实际写的时候发现很多坑。
1.第一步：先不管括号的问题
2.第二步：先不管乘法问题
3.第三步：实现加减运算
4.第四步：解决连续数字问题 

class Solution:
    def solve(self, s: str) -> int:
        num, opt, stack = 0, '+', []
        while s:
            item,s = s[0],s[1:]
            
            if(item=='('):
                lens,end = 1,0
                while lens > 0:
                    if s[end] == '(':
                        lens += 1
                    if s[end] == ')':
                        lens -= 1
                    end += 1
                num = self.solve(s[0:end - 1])
                s = s[end:]

            elif item.isdigit():
                num = num*10+int(item)

            if not item.isdigit() or not s:
                if opt == '+':
                    stack.append(num)
                elif opt == '-':
                    stack.append(-num)
                elif opt =='*':
                    stack.append(stack.pop()*num)
                num,opt =0, item

        return sum(stack)