Group Words by First Letter in Scala

Given an array words of lowercase words, group the words by their first letter. Return a mutable.Map where each key is a first letter, written as a one-character string, and its value is an array of all the words that start with that letter.

Inside each group, keep the words in the order they have in words. The order of the entries in the mutable.Map doesn't matter.

Example 1:

Input: words = ["apple", "banana", "avocado", "blueberry", "cherry"]
Output: {"a": ["apple", "avocado"], "b": ["banana", "blueberry"], "c": ["cherry"]}
Explanation: "apple" and "avocado" start with "a", "banana" and "blueberry" start with "b", and "cherry" is alone under "c".

Example 2:

Input: words = ["dog", "cat", "deer", "cow", "duck"]
Output: {"d": ["dog", "deer", "duck"], "c": ["cat", "cow"]}
Explanation: "dog" comes before "deer" and "duck" in words, so it comes first in the "d" group too.

Example 3:

Input: words = ["hello"]
Output: {"h": ["hello"]}
Explanation: One word makes one group.

Rules:

• words has at least one word.
• Every word has at least one letter and only lowercase letters a-z.
• A word that appears more than once in words appears as many times in its group.