times = 0
def test(num,a,b,c):
    globaltimes
    ifnum==1:
       print (a,b)
       times+=1

    else:
       test(num-1,a,c,b)
       test(1,a,b,c)
       test(num-1,c,b,a)
      

  

test(12,"a","b","c")
print "经过的步数passing：%d"%times