227. Basic-Calculator-II

class Solution:
    def calculate(self, s: str) -> int:
        stack = []
        currentNumber = 0
        sign = 1
        i = 0
        while i < len(s):
            if s[i] in ['+','-']:
                stack.extend([sign, currentNumber])
                currentNumber = 0
                sign = 1 if s[i] == '+' else -1
            elif s[i] in ['*','/']:
                j = i + 1
                nextNumber = 0
                while j < len(s) and (s[j].isdigit() or s[j] == ' '):
                    if s[j].isdigit():
                        nextNumber = nextNumber * 10 + int(s[j])
                    j += 1
                if s[i] == '*':
                    currentNumber *= nextNumber
                else:
                    currentNumber = math.floor(currentNumber / nextNumber)
                i = j - 1
            elif s[i].isdigit():
                currentNumber = currentNumber * 10 + int(s[i])
            if i == len(s) - 1:
                stack.extend([sign, currentNumber])
            i += 1
        res = 0   
        for i in range(0,len(stack),2):
            res += stack[i] * stack[i+1]
        return res


class Solution:
    def calculate(self, s: str) -> int:
        value, n = 0, len(s)

        sign = '+'
        stack = []

        for i, char in enumerate(s):

            if char.isdigit():
                value = value * 10 + int(char)
            
            if i == n - 1 or char in '+-*/':
                if sign == '+':
                    stack.append(value)
                elif sign == '-':
                    stack.append(-value)
                elif sign == '*':
                    stack.append(stack.pop() * value)
                elif sign == '/':
                    stack.append(int(stack.pop() / value))
                
                sign = char
                value = 0
        return sum(stack)