```python
# a = "abc", ոչ դատարկ, եզակի (m)
# s = "dfaga[bfkac]ebf" => "aceb" (n) O(n)
# s = "cfaafb" => "cfaafb"
# s = "affb" => ""
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
```
