核心思想：每一个合数都是由最小质因数和另一个数得到。摆烂式扩充
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def get_primes(n):# get primes in 1 ~ n using euler's method which is O(n)
    check = [1] * (n + 1)
    primes = []
    for i in range(2, n + 1):
        if check[i]:
            primes.append(i)
        for prime in primes:
            if i * prime > n:
                break
            check[i * prime] = 0
            if i % prime == 0: 
                break 
    return primes