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* Pyupgrade to Python 3.9
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53 lines
1.3 KiB
Python
53 lines
1.3 KiB
Python
"""
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The Reverse Polish Nation also known as Polish postfix notation
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or simply postfix notation.
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https://en.wikipedia.org/wiki/Reverse_Polish_notation
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Classic examples of simple stack implementations
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Valid operators are +, -, *, /.
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Each operand may be an integer or another expression.
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"""
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from __future__ import annotations
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from typing import Any
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def evaluate_postfix(postfix_notation: list) -> int:
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"""
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>>> evaluate_postfix(["2", "1", "+", "3", "*"])
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9
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>>> evaluate_postfix(["4", "13", "5", "/", "+"])
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6
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>>> evaluate_postfix([])
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0
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"""
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if not postfix_notation:
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return 0
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operations = {"+", "-", "*", "/"}
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stack: list[Any] = []
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for token in postfix_notation:
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if token in operations:
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b, a = stack.pop(), stack.pop()
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if token == "+":
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stack.append(a + b)
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elif token == "-":
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stack.append(a - b)
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elif token == "*":
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stack.append(a * b)
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else:
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if a * b < 0 and a % b != 0:
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stack.append(a // b + 1)
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else:
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stack.append(a // b)
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else:
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stack.append(int(token))
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return stack.pop()
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if __name__ == "__main__":
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import doctest
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doctest.testmod()
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