oheyandy

collatz.py

import time
bound,mem = 1000000,{}

def collatz(n): 
    if n==1: return [1]
    else: return [n] + collatz(n/2 if (n%2==0) else 3*n+1) if (n>1) else [1] 

def collatzCount(n):
    if n not in mem: mem[n]= 1 + collatzCount(n/2 if (n%2==0) else 3*n+1) if (n>1) else 1
    return mem[n] 

time1 = time.time()
print len(collatz(max(((i,collatzCount(i)) for i in xrange(1,bound)),key=lambda x: x[1])[0]))
print time.time()-time1