An array is called `centered-N` if some `consecutive sequence` of elements of the array sum to `N` and this sequence is preceded and followed by the same number of elements. 
Example:
```
[3,2,10,4,1,6,9] is centered-15
because the sequence 10,4,1 sums to 15 and the sequence 
is preceded by two elements [3,2] and followed by two elements [6,9]
```
Write a method called `isCenteredN` that returns :
- `true` if its array argument is `not empty` and `centered-N` or empty and centered-0
- otherwise returns `false`.
def is_centered(arr, n):
    x = sum(arr)
    if x == n: return True
    for i in range(len(arr)>>1):
        x -= arr[i] + arr[-i-1]
        if x == n: return True
    return False
def is_centered(arr, n):
    x = sum(arr)
    if x == n: return True
    for i in range(len(arr)>>1):
        x -= arr[i] + arr[-i-1]
        if x == n: return True
    return False