I found an interesting problem on https://www.codewars.com/kata/59b844528bcb7735560000a0:
A `Nice array` is defined to be an array where for every value `n` in the array, there is also an element `n-1` or `n+1` in the array.

example:

```
[2,10,9,3] is Nice array because

2=3-1
10=9+1
3=2+1
9=10-1
```
Write a function named `isNice`/`IsNice` that returns `true` if its array argument is a Nice array, else `false`. You should also return `false` if `input` array has `no` elements.

I tried it in Python, but could not do it. Can you solve it?
def is_nice(arr):
    s = set(arr)
    return bool(arr) and all( n+1 in s or n-1 in s for n in s)