Sliding Window Target: 15s
Counter-based fixed window slides character frequencies. Powers anagram detection.
from collections import Counter
w = Counter(s[:k])
for i in range(k, len(s)):
w[s[i]] += 1
w[s[i-k]] -= 1
if w[s[i-k]] == 0: del w[s[i-k]]Type it from memory. Go.