Below is the code i have written which only check the 1st and the last patter and not in the middle. 
Any help is much appreciated. 

```
def arith_geo(list_n):
    if len(list_n) == 0:
        return 0

    for n in range(len(list_n) - 1): 
        if (list_n[n + 1] - list_n[n]) == (list_n[n-1] - list_n[n-2]):
            return 'Arithmetic'
        elif (list_n[n + 1] / list_n[n]) == (list_n[n -1 ] / list_n[n - 2]):
            return 'Geometric'
        else:
            return -1
print(arith_geo([1, 2, 3, 8, 5, 6]))

```