Basic Calculator II LeetCode solution in Python

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# Runtime: 201 ms (Top 18.42%) | Memory: 15.8 MB (Top 36.40%)
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class Solution:
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    def calculate(self, s: str) -> int:
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        stack = []
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        currentNumber = 0
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        operator = "+"
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        operations = "+-/*"
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        for i in range(len(s)):
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            ch = s[i]
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            if ch.isdigit():
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                currentNumber = currentNumber * 10 + int(ch)
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            if ch in operations or i == len(s) - 1:
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                if operator == "+":
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                    stack.append(currentNumber)
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                elif operator == "-":
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                    stack.append(-currentNumber)
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                elif operator == "*":
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                    stack.append(stack.pop() * currentNumber)
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                elif operator == "/":
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                    stack.append(int(stack.pop() / currentNumber))
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                currentNumber = 0
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                operator = ch
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        return sum(stack)