def collatz_sequence(n):
"""Collatz conjecture: start with any positive integer n.Next termis obtained from the previous term as follows:
if the previous term is even, the next term is one half the previous term.
If the previous term is odd, the next term is 3 times the previous term plus 1.
The conjecture states the sequence will always reach 1 regaardess of starting n."""
sequence = [n]
while n != 1:
if n % 2 == 0:# even
n //= 2
else:
n = 3*n +1
sequence.append(n)
return sequence
answer = max([(len(collatz_sequence(i)), i) for i in range(1,1000000)])
print("Longest Collatz sequence under one million is %d with length %d" % (answer[1],answer[0]))