feat(challenges): add frequency-deviation

challenges/frequency-deviation/README.md

@@ -0,0 +1,67 @@
+# frequency-deviation
+
+Created by [@theoludwig](https://github.com/theoludwig) on 16 September 2023.
+
+## Instructions
+
+Given a string consisting of lowercase English letters, we define the frequency deviation of a substring as the difference between the maximum and the minimum frequencies of the characters in that substring.
+
+A substring of a string is formed by any contiguous segment of the string. For example, given "bbacccc", the character appearing most frequently is 'c' with $4$ occurrences. The character that appears the fewest times is 'a' with $1$ occurrence. The frequency deviation of the entire string is $4 - 1 = 3$.
+
+Given a string, $s$, representing the input string, find the maximum possible frequency deviation of any of its substrings.
+
+### Constraints
+
+- $$1 \leq s.length \leq 10^4$$
+- $s$ consists of lowercase English letters.
+
+## Source
+
+- [LeetCode - Substring With Largest Variance](https://leetcode.com/problems/substring-with-largest-variance/)
+- [Twitter @CoderNolimit](https://twitter.com/CoderNolimit/status/1668147202173050881)
+
+## Examples
+
+See the `test` folder for examples of input/output.
+
+### Example 1
+
+#### Input
+
+```txt
+bbacccc
+```
+
+#### Output
+
+```txt
+3
+```
+
+### Example 2
+
+#### Input
+
+```txt
+aabb
+```
+
+#### Output
+
+```txt
+1
+```
+
+### Example 3
+
+#### Input
+
+```txt
+aaaaa
+```
+
+#### Output
+
+```txt
+0
+```