TheAlgorithms-Python

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"""
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Project Euler Problem 2: https://projecteuler.net/problem=2
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Even Fibonacci Numbers
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Each new term in the Fibonacci sequence is generated by adding the previous
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two terms. By starting with 1 and 2, the first 10 terms will be:
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1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
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By considering the terms in the Fibonacci sequence whose values do not exceed
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four million, find the sum of the even-valued terms.
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References:
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    - https://en.wikipedia.org/wiki/Fibonacci_number
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"""
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def solution(n: int = 4000000) -> int:
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    """
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    Returns the sum of all even fibonacci sequence elements that are lower
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    or equal to n.
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    >>> solution(10)
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    10
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    >>> solution(15)
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    10
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    >>> solution(2)
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    2
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    >>> solution(1)
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    0
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    >>> solution(34)
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    44
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    """
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    if n <= 1:
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        return 0
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    a = 0
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    b = 2
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    count = 0
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    while 4 * b + a <= n:
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        a, b = b, 4 * b + a
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        count += a
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    return count + b
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if __name__ == "__main__":
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    print(f"{solution() = }")
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