Recover a secret string from random triplets

recover.py

from itertools import chain


def get_behind_chars(c, pool):
    behind = []
    for item in pool:
        if c in item:
            chars = item[item.index(c):]
            [behind.append(_) for _ in chars if _ != c]
    return list(set(behind))


def get_pos(word, triplets):
    levels = []
    for level in range(len(triplets[0]) + 1):
        if not levels:
            levels.append(get_behind_chars(word, triplets))
        else:
            levels.append([c for char in levels[level - 1] for c in get_behind_chars(char, triplets)])
    return len(set(chain(*levels)))


def recoverSecret(triplets):
    flat = set(c for triplet in triplets for c in triplet)
    words = {word: get_pos(word, triplets) for word in flat}
    order = sorted(words.items(), key=lambda kv: kv[1])
    return ''.join([tup[0] for tup in order][::-1])