- Input: Integer `n`
- Output: String
Example:
`a(4)` prints as
```
   A   
  A A  
 A A A 
A     A
```
`a(8)` prints as
```
       A       
      A A      
     A   A     
    A     A    
   A A A A A   
  A         A  
 A           A 
A             A
```
`a(12)` prints as
```
           A           
          A A          
         A   A         
        A     A        
       A       A       
      A         A      
     A A A A A A A     
    A             A    
   A               A   
  A                 A  
 A                   A 
A                     A
```
Note:
- Each line's length is `2n - 1`
- Each line should be concatenate by line break `"\n"`
- If `n` is less than `4`, it should return `""`
- If `n` is odd, `a(n) = a(n - 1)`, eg `a(5) == a(4); a(9) == a(8)`
def a(n):
    n = n if n % 2 == 0 else n - 1
    width = 2 * n - 1
    lines = ["A".center(width)] + [("A" + " "*k + "A").center(width) for k in range(1, n-1, 2)] +\
        [" ".join(["A"]*(n//2+1)).center(width)] + [("A" + " "*k + "A").center(width) for k in range(n+1, 2*n-1, 2)]
    return "\n".join(lines) if n >= 4 else ""
def a(n):
    n = n if n % 2 == 0 else n - 1
    width = 2 * n - 1
    lines = ["A".center(width)] + [("A" + " "*k + "A").center(width) for k in range(1, n-1, 2)] +\
        [" ".join(["A"]*(n//2+1)).center(width)] + [("A" + " "*k + "A").center(width) for k in range(n+1, 2*n-1, 2)]
    return "\n".join(lines) if n >= 4 else ""