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Project Euler 016, 020を解いてみる。「各位の数字の和」

Last updated at Posted at 2019-09-14

Project Euler 016, 020

016

215 = 32768 であり, 各位の数字の和は 3 + 2 + 7 + 6 + 8 = 26 となる.
同様にして, 21000 の各位の数字の和を求めよ.

020

n × (n - 1) × ... × 3 × 2 × 1 を n! と表す.
例えば, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800 となる.
この数の各桁の合計は 3 + 6 + 2 + 8 + 8 + 0 + 0 = 27 である.
では, 100! の各位の数字の和を求めよ.

->次の問題

考え方

016と020は類似の問題なので合わせて解きます。
桁数や第1桁、最高位の数字を出す方法はありますが、各位の数字は実際に計算する以外の方法が思いつかなかったので、単純に計算しています。
pythonはstrにすることで各桁の数字を文字列で取り出せるので便利ですね。

コード

euler016.py
def main016():
    number = 2 ** 1000
    num_list = [int(i) for i in str(number)]
    print(sum(num_list))


def main020():
    number = 1
    for i in range(2, 101):
        number *= i
    num_list = [int(i) for i in str(number)]
    print(sum(num_list))


if __name__ == '__main__':
    from time import time as t
    start = t()
    main016()
    print(t() - start)
    start = t()
    main020()
    print(t() - start)

paizaにて実行
結果 1366 6.604194641113281e-05 648 3.266334533691406e-05
シンプルな問題が続きますね

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