Basic Calculator II Python Solution with Tests

lcpy gen -n 227   # by problem number
lcpy gen -s basic_calculator_ii   # by problem name

Input: s = "3+2*2"
Output: 7

Input: s = " 3/2 "
Output: 1

Input: s = " 3+5 / 2 "
Output: 5

class Solution:
    # Time: O(n) — single pass
    # Space: O(n) — stack of operands
    def calculate(self, s: str) -> int:
        stack: list[int] = []
        current = 0
        op = "+"

        def apply(value: int) -> None:
            nonlocal stack
            if op == "+":
                stack.append(value)
            elif op == "-":
                stack.append(-value)
            elif op == "*":
                stack.append(stack.pop() * value)
            else:  # "/"
                prev = stack.pop()
                # Truncate toward zero
                stack.append(int(prev / value))

        for ch in s:
            if ch.isdigit():
                current = current * 10 + int(ch)
            elif ch in "+-*/":
                apply(current)
                op = ch
                current = 0

        apply(current)  # last operand
        return sum(stack)