My first Rust program - Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent.

phone-number-combinations.rs

// https://leetcode.com/problems/letter-combinations-of-a-phone-number/description/
use std::collections::HashMap;
use std::char;
use std::iter::FromIterator;

fn main() {
    let mappings = init_map();
    find_combinations("23", mappings)
}

fn init_map() -> HashMap<i32, Vec<char>> {
    let mut number_to_letters = HashMap::new();
    let a_ord = 96; // char 'a'
    let mut curr_ord = a_ord;
    for n in 0..8 {
        let mut letters = Vec::new();
        let total = if n + 2 == 7 || n + 2 == 9 { 4 } else { 3 };
        for i in 0..total {
            letters.push(char::from_u32(curr_ord + i + 1).unwrap());
        }
        number_to_letters.insert(n + 2, letters);
        curr_ord += total;
    }
    return number_to_letters;
}

fn find_combinations(input: &str, mappings: HashMap<i32, Vec<char>>) {
    const RADIX: u32 = 10;
    let all_letters: Vec<&Vec<char>> = input
        .chars()
        .map(|c| c.to_digit(RADIX).unwrap())
        .map(|n| mappings.get(&(n as i32)).unwrap())
        .collect();
    let combinations = permutation(&mut vec![], all_letters);
    println!("{:?}", combinations);
}

fn permutation<'a>(curr: &mut Vec<char>, letter_vectors: Vec<&'a Vec<char>>) -> Vec<String> {
    if letter_vectors.is_empty() {
        return vec![String::from_iter(&*curr)];
    }
    let mut result = Vec::new();
    if let Some((first, rest)) = letter_vectors.split_first() {
        for letter in first.iter() {
            curr.push(*letter);
            result.append(&mut permutation(curr, rest.to_vec()));
            curr.pop();
        }
    }
    return result;
}