Here your task is to Create a (nice) Christmas Tree.
You don't have to check errors or incorrect input values, every thing is ok without bad tricks, only one int parameter as input and a string to return;-)...
So what to do?First three easy examples:

````
Input: 3 and Output:
  *
 ***
*****
 ###
 
 Input 9 and Output:
    *
   ***
  *****
   ***
  *****
 *******
  *****
 *******
*********
   ###
   
 Input 17 and Output:
      *
     ***
    *****
     ***
    *****
   *******
    *****
   *******
  *********
   *******
  *********
 ***********
  *********
 ***********
*************
     ###
     
Really nice trees, or what???! So merry Christmas;-)
````

You can see, always a root, always steps of hight 3, tree never smaller than 3 (return "") and no difference for input values like 15 or 17 (because (int) 15/3 = (int) 17/3). That's valid for every input and every tree.
I think there's nothing more to say - perhaps look at the testcases too;-)!

There are some static tests at the beginning and many random tests if you submit your solution. 
Hope you have fun:-)!

Difficulty: introductory

Starter code:

def christmas_tree(height):
	

Solutions:
def christmas_tree(h):
  return "" if h<3 else "\r\n".join(["\r\n".join([" "*(((5-y)//2)+(h//3)-i-1)+"*"*(y+i*2) for y in [1,3,5]]) for i in range(h//3)])+"\r\n"+" "*(h//3)+"###"

def christmas_tree(height):
    height, tree = height // 3, []
    for floor in range(height):
        for branch in range(floor, 3 + floor):
            tree.append(f"{' ' * (height - branch + 1)}{'*' * (branch * 2 + 1)}")
    tree.append(f"{' ' * (height)}###")
    return "\r\n".join(tree) if height else ""

def christmas_tree(height):
    height = height // 3 * 3
    if height <= 0:
        return ""
    lines = []
    w = 3
    for i in range(height):
        w += 2 if i % 3 else -2
        lines.append("*" * w)
    lines.append("###")
    return "\r\n".join(line.center(w).rstrip() for line in lines)

def christmas_tree(height):
    if height < 3:
        return ""
    result = []
    for i in range(height // 3):
        w = 1 + 2 * i
        result.append("*" * w)        
        result.append("*" * (w + 2))
        result.append("*" * (w + 4))
    result.append("###")
    l = len(result[-2])
    return "\r\n".join([line.center(l).rstrip() for line in result])

christmas_tree=lambda n:'\r\n'.join([(n//3-j+1)*' '+(2*j+1)*'*'for i in range(n//3)for j in range(i,i+3)]+[n//3*' '+3*'#']*(n//3>0))

christmas_tree=lambda n:"\r\n".join([('*'*(j-p)).center((n//3)*2+3," ").rstrip()for o,i in enumerate(range(1,(n//3)*3,3))for p,j in enumerate(range(i-o,(i-o)+9,3))]+['###'.center((n//3)*2+3).rstrip()]if n>2else'')

def christmas_tree(height):
    h = height - height % 3
    n, r = h // 3, ''
    sz, l1, l2, l3, k = 5 + (n - 1) * 2, '*', '***', '*****', '\r\n'
    w = (sz - len(l1)) // 2
    for i in range(n):
        r += ' ' * w + l1 + k
        r += ' ' * (w-1) + l2 + k        
        r += ' ' * (w-2) + l3 + k
        w -= 1
        l1 += '**'
        l2 += '**'
        l3 += '**'
    return r + ' ' * ((sz - 3) // 2) + '###' if height>2 else ''


def christmas_tree(height):
    s = '* *** *****'.split()
    r = []
    for i in range(height//3):
        for x in s:
            r.append(' '*((2*(height//3)+3-2*i-len(x))//2) + '*'*2*i + x)
    return '\r\n'.join(r+[' '*(height//3)+'###']) if height//3 else ''

def christmas_tree(height):
    if height < 3:
        return ''
    res = [1, 3, 5]
    for _ in range((height-3)//3):
        res.extend(res[-2:] + [res[-1]+2])
    w = max(res)
    ans = [('*' * r).center(w).rstrip() for r in res] + ['###'.center(w).rstrip()]
    return '\r\n'.join(ans)

def christmas_tree(h):
    if h < 3: return ""
    h, r = h // 3, []
    m = h * 2 + 3
    r = ["*" * (2 * (i + j) + 1) for i in range(h) for j in range(3)] + ["#" * 3]
    return "\r\n".join(map(lambda x: x.center(m).rstrip(), r))