The function takes a string as input and performs one of the following operations, based on the input:
- If the input is "12345", it reverses the string.
- If the input is a lowercase string, it capitalizes the string.
- If the input is a sentence, it removes all spaces from the string.
- If the input is anything other than the above, it returns "Invalid input!"

[CODE]:
```python
def f(x):
    if x == "12345":
        return x[::-1]
    elif x.islower():
        return x.upper()
    elif " " in x:
        return x.replace(" ", "")
    else:
        return "Invalid input!"
```