The function returns "one" if there is an exclamation mark at the beginning or the end of the input string, and "zero" for all other cases, including exclamation marks in the middle of the string or no exclamation marks.

[CODE]:
```python
def approx_function(input_string):
    if input_string.startswith('!') or input_string.endswith('!'):
        return 'one'
    else:
        return 'zero'
```
