Python solution for https://projecteuler.net/problem=14

collatz.py

def collatz(n, seq):
  seq.append(n)

  if n == 1:
    return n
  else:
    if n % 2 == 0:
      return collatz(int(n/2), seq)
    else:
      return collatz(((3*n)+1), seq)
      
def main():
  longestLen = 0
  longestSeq = 0
  for n in range(1, 1000000):
    seq = list()
    collatz(n, seq)
    if len(seq) > longestLen:
      longestLen = len(seq)
      longestSeq = seq
  
  print "The longest seq is %s with a length of %s." % (longestSeq, longestLen)
  
main()