Distinct split

Let $f(x)$ be the number of distinct letters in string $x$. Split a string $s$ of lowercase Latin letters into two non-empty strings $a$ and $b$ ($a + b = s$) so that $f(a) + f(b)$ is as large as possible. Find this maximum value.

The first line contains $n$ ($2 \le n \le 2 \cdot 10^5$). The second line contains a string $s$ of $n$ lowercase Latin letters.

A single number — the maximum value of $f(a) + f(b)$.

Recounting both parts for every split point is $O(n^2)$. Instead compute $L_i$ — the number of distinct letters among the first $i$ (prefix) and $R_i$ — the number of distinct letters from position $i$ to the end (suffix). Both arrays take one pass each, and the answer is $\max L_i + R_{i+1}$.

In the first example abc + abcd: $3 + 4 = 7$.

Source: Codeforces, problem 1791D.