451. Sort Characters By Frequency

Input:
"tree"

Output:
"eert"

Explanation:
'e' appears twice while 'r' and 't' both appear once.
So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.

Input:
"cccaaa"

Output:
"cccaaa"

Explanation:
Both 'c' and 'a' appear three times, so "aaaccc" is also a valid answer.
Note that "cacaca" is incorrect, as the same characters must be together.

Input:
"Aabb"

Output:
"bbAa"

Explanation:
"bbaA" is also a valid answer, but "Aabb" is incorrect.
Note that 'A' and 'a' are treated as two different characters.

Rust Solution

struct Solution;
use std::cmp::Reverse;
use std::collections::HashMap;

impl Solution {
    fn frequency_sort(s: String) -> String {
        let mut s: Vec<char> = s.chars().collect();
        let mut hm: HashMap<char, usize> = HashMap::new();
        for &c in &s {
            *hm.entry(c).or_default() += 1;
        }
        s.sort_unstable_by_key(|&c| (Reverse(hm[&c]), Reverse(c)));
        s.into_iter().collect()
    }
}

#[test]
fn test() {
    let s = "tree".to_string();
    let res = "eetr".to_string();
    assert_eq!(Solution::frequency_sort(s), res);
    let s = "cccaaa".to_string();
    let res = "cccaaa".to_string();
    assert_eq!(Solution::frequency_sort(s), res);
    let s = "Aabb".to_string();
    let res = "bbaA".to_string();
    assert_eq!(Solution::frequency_sort(s), res);
}