```python # a = "abc", not-empty, unique (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 ```