def solution(max): s_len = 0 s_num = 0 lengths = [] for i in range(0, max): length = 0 num = i while num > 0: length += 1 if num == 1: break if num % 2 == 0: num = num / 2 else: num = (3 * num) + 1 if num < len(lengths): length += lengths[num] num = 0 lengths.append(length) if length > s_len: s_len = length s_num = i print('n:%s\tlen:%s' % (s_num, s_len)) print(len(collatz(s_num))) def collatz(start): seq = [] num = start while num > 0: seq.append(num) if num == 1: break if num % 2 == 0: num = num / 2 else: num = (3 * num) + 1 return seq if __name__ == '__main__': solution(1000000)