Take 2 strings `s1` and `s2` including only letters from `a`to `z`.
Return a new **sorted** string, the longest possible, containing distinct letters,
- each taken only once - coming from s1 or s2.
# Examples:
```
a = "xyaabbbccccdefww"
b = "xxxxyyyyabklmopq"
longest(a, b) -> "abcdefklmopqwxy"
a = "abcdefghijklmnopqrstuvwxyz"
longest(a, a) -> "abcdefghijklmnopqrstuvwxyz"
```
def longest(s1, s2):
    letters_count = [0] * 26
    for letter in s1:
        letters_count[ord(letter) - 97] += 1
    for letter in s2:
        letters_count[ord(letter) - 97] += 1
    result = []
    for i in range(26):
        if letters_count[i] > 0:
            result.append(chr(i+97))
    return ''.join(result)
def longest(s1, s2):
    letters_count = [0] * 26
    for letter in s1:
        letters_count[ord(letter) - 97] += 1
    for letter in s2:
        letters_count[ord(letter) - 97] += 1
    result = []
    for i in range(26):
        if letters_count[i] > 0:
            result.append(chr(i+97))
    return ''.join(result)