pub fn letter_combinations(digits: String) -> Vec<String> {

if digits.len() == 0 {

return vec![];

}

let mut ans = vec!["".to_string()];

let map: HashMap<char, Vec<char>> = [

('2', vec!['a', 'b', 'c']),

('3', vec!['d', 'e', 'f']),

('4', vec!['g', 'h', 'i']),

('5', vec!['j', 'k', 'l']),

('6', vec!['m', 'n', 'o']),

('7', vec!['p', 'q', 'r', 's']),

('8', vec!['t', 'u', 'v']),

('9', vec!['w', 'x', 'y', 'z'])

].iter().cloned().collect();

for d in digits.chars() {

let mut tmp = vec![];

if let Some(v) = map.get(&d) {

for s in &ans {

for ch in v {

tmp.push(s.clone() + &ch.to_string());

}

}

}

ans = tmp;

}

ans

}