gistfile1.py

# by aji, http://github.com/aji
# (couldn't be bothered to log into account at work)

import sys

alphabet = sys.argv[1]
length = int(sys.argv[2])

# yields every possible permutation of n elements of alphabet
def each(n=length):
    if n == 0:
        yield ''
        return
    for x in each(n-1):
        for y in alphabet:
            yield x + y

# yields every node adjacent to s in the graph
def adj(s):
    for x in alphabet:
        yield s[1:] + x

# find the first winning sequence of nodes
def dfs(seen, depth_want=None):
    if depth_want is None:
        depth_want = len(alphabet) ** len(seen[0])
    if len(seen) == depth_want:
        return seen
    for n in adj(seen[-1]):
        if n not in seen:
            winner = dfs(seen + [n], depth_want)
            if winner is not None:
                return winner
    return None

# invoke
winner = dfs([alphabet[0] * length])
s = ''.join([winner[0][:-1]] + [x[-1] for x in winner])

# output
print(s)
for x in each():
    if x not in s:
        print('alg missed ' + x)