# Task
Christmas is coming. In the [previous kata](https://www.codewars.com/kata/5a405ba4e1ce0e1d7800012e), we build a custom Christmas tree with the specified characters and the specified height.
Now, we are interested in the center of the Christmas tree. 
Please imagine that we build a Christmas tree with `chars = "abc" and n = Infinity`, we got:
```
          a
         b c
        a b c
       a b c a
      b c a b c
     a b c a b c
    a b c a b c a
   b c a b c a b c
  a b c a b c a b a
 b c a b c a b a b c
a b c a b a b c a b c
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
          |
          |
          .
          .
```
If we keep only the center part of leaves, we will got:
```
          a
          b  
          a    
          a        
          b        
          a          
          .
          .
          .
```
As you can see, it's a infinite string, but it has a repeat substring "aba"(spaces will be removed). If we join them together, it looks like:`"abaabaabaabaaba......"`.
So, your task is to find the repeat substring of the center part of leaves.
# Inputs:
- `chars`: the specified characters. In this kata, they always be lowercase letters.
# Output:
- The repeat substring that satisfy the above conditions.
Still not understand the task? Look at the following example ;-)
# Examples
For `chars = "abc"`,the output should be `"aba"`
```
center leaves sequence: "abaabaabaabaabaaba....."
```
For `chars = "abab"`,the output should be `a`
```
center leaves sequence: "aaaaaaaaaa....."
```
For `chars = "abcde"`,the output should be `aecea`
```
center leaves sequence: "aeceaaeceaaecea....."
```
For `chars = "aaaaaaaaaaaaaa"`,the output should be `a`
```
center leaves sequence: "aaaaaaaaaaaaaa....."
```
For `chars = "abaabaaab"`,the output should be `aba`
```
center leaves sequence: "abaabaabaaba....."
```
def center_of(chars):
    if chars == '':
        return ''
    chars = chars * 10000
    def n_int(n):
        return n*(n+1) // 2
    center = ''
    c = 0
    i= 0
    while n_int(i) <= len(chars):
        if i%2 != 0:
            c += 1
            pos_center = (n_int(i) - c) 
            center += chars[pos_center]
        i += 1
    for x in range(1, len(center)):
        subcenter = center[:x]
        if subcenter * (len(center)//len(subcenter))+(subcenter[:len(center)%len(subcenter)]) == center:
            return(subcenter)
def center_of(chars):
    if chars == '':
        return ''
    chars = chars * 10000
    def n_int(n):
        return n*(n+1) // 2
    center = ''
    c = 0
    i= 0
    while n_int(i) <= len(chars):
        if i%2 != 0:
            c += 1
            pos_center = (n_int(i) - c) 
            center += chars[pos_center]
        i += 1
    for x in range(1, len(center)):
        subcenter = center[:x]
        if subcenter * (len(center)//len(subcenter))+(subcenter[:len(center)%len(subcenter)]) == center:
            return(subcenter)