Given a long number, return all the possible sum of two digits of it.
For example, `12345`: all possible sum of two digits from that number are:
    [ 1 + 2, 1 + 3, 1 + 4, 1 + 5, 2 + 3, 2 + 4, 2 + 5, 3 + 4, 3 + 5, 4 + 5 ]
Therefore the result must be:
    [ 3, 4, 5, 6, 5, 6, 7, 7, 8, 9 ]
def digits(num):
    num = str(num)
    return [int(x)+int(y) for n, x in enumerate(num,1) for y in num[n:]]

def digits(num):
    num = str(num)
    return [int(x)+int(y) for n, x in enumerate(num,1) for y in num[n:]]
