Good subarrays

You are given an array $a_1, \ldots, a_n$ of digits from $0$ to $9$. A subarray $a_l, \ldots, a_r$ is good if the sum of its elements equals its length: $\sum_{i=l}^{r} a_i = r - l + 1$. Count the good subarrays.

The first line contains $n$ ($1 \le n \le 10^5$). The second line contains a string of $n$ digits (no spaces).

A single number — the number of good subarrays.

Subtract $1$ from every element: $b_i = a_i - 1$. Now the condition is $\sum_{i=l}^{r} b_i = 0$, that is, two prefixes are equal: $q_r = q_{l-1}$. Count how many times every prefix value occurs — a value seen $c$ times gives $\frac{c(c-1)}{2}$ pairs. The prefixes lie in $[-n, 8n]$, so a shifted array works instead of a dictionary.

In the first example 1, 20 and 120 are good. If all digits are 1, the answer is $\frac{n(n+1)}{2}$ — a 64-bit type is needed.

Source: Codeforces, problem 1398C.