Given a string, sort it in decreasing order based on the frequency of characters.
Example 1:
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.
Example 2:
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.
Example 3:
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.
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);
}