Letter Count in Rust

Given a string word, return a HashMap that maps each letter of word to the number of times it appears in word.

Every letter of word is a key, and its value is how many times that letter appears. The order of the entries doesn't matter.

Example 1:

Input: word = "banana"
Output: {"b": 1, "a": 3, "n": 2}
Explanation: "b" appears once, "a" three times and "n" twice.

Example 2:

Input: word = "hello"
Output: {"h": 1, "e": 1, "l": 2, "o": 1}
Explanation: "l" appears twice; every other letter appears once.

Example 3:

Input: word = "a"
Output: {"a": 1}
Explanation: A single letter appears once.

Constraints:

• word has at least one letter.
• word is made of lowercase English letters, a to z.
• Only the letters of word are keys: a letter that doesn't appear has no entry.