```python
from collections import Counter
def min_s(a: str, s: str) -> str:
need = set(a)
window = Counter()
res = ""
c = 0
left = 0
best_len = float('inf')
for right, char in enumerate(s):
if char in need and \
window[char] += 1
if window[char]:
c += 1
while c == len(need):
if right - left + 1< best_len:
res = s[left:right + 1]
best_len = right - left + 1
if s[left] in need:
window[s[left]] -= 1
if window[s[left]] == 0:
c -= 1
left += 1
return res
```
Úlohou je implementovať funkciu `min_s(a: str, s: str) -> str`, ktorá nájde najmenší podreťazec v `s`, ktorý obsahuje všetky znaky z reťazca `a`.
