Facebook Hacker Cup Rount1 #2 Security

Security.py

import sys

def compare(chunk_key1, chunk_key2):
    answer = []
    for key1, key2 in zip(chunk_key1, chunk_key2):
        for c1, c2 in zip(key1, key2):
            if c1 == '?' and c2 == '?':
                answer.append('a')
            elif c1 == '?':
                answer.append(c2)
            elif c2 == '?':
                answer.append(c1)
            elif c1 == c2:
                answer.append(c1)
            else:
                return "IMPOSSIBLE"
    return "".join(answer)

def splitToChunk(in_str, m):
    l = len(in_str) / m
    return [in_str[i*l:(i+1)*l] for i in xrange(m)]

def un_function_g(chunk):
    item = chunk.pop()
    chunk.insert(0, item)
    return chunk

def main():
    T = int(sys.stdin.readline().rstrip())
    for i in xrange(T):
        m = int(sys.stdin.readline().rstrip())
        key1 = sys.stdin.readline().rstrip()
        key2 = sys.stdin.readline().rstrip()
        chunk_key1 = splitToChunk(key1, m)
        chunk_key2 = splitToChunk(key2, m)
        converted_chunk_key2 = un_function_g(chunk_key2)
        print("Case #{0:d}: {1:s}".format(i+1, compare(
            chunk_key1, converted_chunk_key2)))

if __name__ == "__main__":
    main()