# Task
 You are given a string `s`. Every letter in `s` appears once. 
 Consider all strings formed by rearranging the letters in `s`. After ordering these strings in dictionary order, return the middle term. (If the sequence has a even length `n`, define its middle term to be the `(n/2)`th term.)
# Example
 For `s = "abc"`, the result should be `"bac"`.
```
The permutations in order are:
"abc", "acb", "bac", "bca", "cab", "cba"
So, The middle term is "bac".```
# Input/Output
 - `[input]` string `s`
  unique letters (`2 <= length <= 26`)
 - `[output]` a string
  middle permutation.
def middle_permutation(s):
    s = ''.join(sorted(s))
    return s[len(s)//2-1:(len(s)+1)//2][::-1] + s[(len(s)+1)//2:][::-1] + s[:len(s)//2-1][::-1]
def middle_permutation(s):
    s = ''.join(sorted(s))
    return s[len(s)//2-1:(len(s)+1)//2][::-1] + s[(len(s)+1)//2:][::-1] + s[:len(s)//2-1][::-1]