Yes. It's a neat trick, but from an algorithmic point of view it's exactly the same as the obvious solution, replacing a set with a hash-set. I did this with a

Yes. It's a neat trick, but from an algorithmic point of view it's exactly the same as the obvious solution, replacing a set with a hash-set. I did this with a counter in python: def first_diff(s, n): cnt = Counter() for i, c in enumerate(s, 1): cnt[c] += 1 if i > n: top = s[i - n - 1] cnt[top] -= 1 if cnt[top] == 0: del cnt[top] if len(cnt) == n: return i