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String Calculator without parenthesis

def calculate(s):
    return helper(s)

def helper(s, index):
    ops = {"+", "-", "/", "*", "#"}
    sign = '+'
    num = 0
    stack = []
    i = index
    s = s + "#"
    while i < len(s):
        if s[i].isdigit():
            num = num*10 + int(s[i])
        if s[i] in ops:

            if sign == "+":
                stack.append(num)
            elif sign == '-':
                stack.append(-num)
            elif sign == "*":
                tmp = stack.pop()
                print(tmp, num, tmp * num)
                stack.append(tmp*num)
            elif sign == "/":
                tmp = stack.pop()
                stack.append(tmp // num)
            sign = s[i]
            # mistake
            num = 0
        i += 1
    return sum(stack)

String Calculator with parenthesis

def calculate(s):
    s = s + "#"
    _, _sum = helper(s, 0)
    return _sum

def helper(s, i):
    stack = []
    sign = "+"
    num = 0
    while i < len(s):
        if s[i] == " ":
            pass
        elif s[i].isdigit():
            num = num*10 + int(s[i])
        elif s[i] == "(":
            i, num = helper(s, i + 1)
        else:
            if sign == "+":
                stack.append(num)
            elif sign == "-":
                stack.append(-num)
            elif sign == "*":
                prev = stack.pop()
                stack.append(prev * num)
            elif sign == "/":
                prev = stack.pop()
                stack.append(prev / num)
            if s[i] == ")":
                return i, sum(stack)
            sign = s[i]
            num = 0
        i += 1
    return i, sum(stack)