Given few numbers, you need to print out the digits that are not being used.
Example:
```python
unused_digits(12, 34, 56, 78) # "09"
unused_digits(2015, 8, 26) # "3479"
```
Note:
- Result string should be sorted
- The test case won't pass Integer with leading zero
def unused_digits(*l):
    digits_set = set(list("0123456789"))
    test_set = set("".join(str(i) for i in l))
    d = digits_set.difference(test_set)
    r = "".join(sorted(d))
    return r
def unused_digits(*l):
    digits_set = set(list("0123456789"))
    test_set = set("".join(str(i) for i in l))
    d = digits_set.difference(test_set)
    r = "".join(sorted(d))
    return r