struct Solution; impl Solution { pub fn frequency_sort(s: String) -> String { let mut map = std::collections::HashMap::with_capacity(26 * 2); s.chars().for_each(|b| * map.entry(b).or_insert(0) += 1); let mut v = map.into_iter().map(|(c, n)| (c, n)).collect::>(); v.sort_unstable_by_key(|pair| - pair.1); v.into_iter().flat_map(|(c, n)| vec![c; n as usize]).collect() } } fn main() { let s = String::from("tree"); print!("{}", Solution::frequency_sort(s)); }