Find all permutations of letters given a phone number

main.rs

use lazy_static::lazy_static;
use std::collections::HashMap;

lazy_static! {
    static ref NUM_MAP: HashMap<char, Vec<char>> = {
        let mut m = HashMap::new();
        m.insert('2', vec!['a', 'b', 'c']);
        m.insert('3', vec!['d', 'e', 'f']);
        m.insert('4', vec!['g', 'h', 'i']);
        m.insert('5', vec!['j', 'k', 'l']);
        m.insert('6', vec!['m', 'n', 'o']);
        m.insert('7', vec!['p', 'q', 'r', 's']);
        m.insert('8', vec!['t', 'u', 'v']);
        m.insert('9', vec!['w', 'x', 'y', 'z']);
        m
    };
}

fn main() {
    println!("{:?}", phone_letter(String::from("23")))
}

fn phone_letter(num: String) -> Vec<String> {
    let mut permuts = vec![String::from("")];

    for c in num.chars() {
        let mut tmp: Vec<String> = vec![];
        for p in permuts.into_iter() {
            let options = NUM_MAP.get(&c).map_or(vec![], |v| v.to_owned());
            for i in options {
                let mut copy = p.clone();
                copy.push(i);
                tmp.push(copy);
            }
        }
        permuts = tmp;
    }

    permuts
}