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From: Ian Kelly <ian.g.kelly@gmail.com>
Date: Tue, 19 Feb 2013 11:31:24 -0700
Subject: Re: Python 3.3 vs. MSDOS Basic
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On Tue, Feb 19, 2013 at 7:46 AM, Tim Daneliuk <tundra@tundraware.com> wrote:
> Are you sure you wouldn't like to share with the class?  I'd be interested
> in seeing your approach...

Very well:

def collatz(n, memo):
    if n not in memo:
        if n % 2 == 0:
            next_n = n // 2
        else:
            next_n = 3 * n + 1
        memo[n] = collatz(next_n, memo) + 1
    return memo[n]

def run_collatz(upper):
    table = {1: 0}
    max_n = max(range(1, upper), key=lambda n: collatz(n, table))
    return max_n, table[max_n]

>>> run_collatz(1000000)
(837799, 524)

It could certainly be optimized further, but at about 4 seconds it's
already fast enough for most purposes.