rs: add «451. Sort Characters By Frequency»

Signed-off-by: Matej Focko <mfocko@redhat.com>

rs/2024-02-07-sort-characters-by-frequency.rs

@@ -0,0 +1,27 @@
+use std::cmp;
+use std::collections::HashMap;
+
+impl Solution {
+    fn get_freqs(s: &str) -> HashMap<char, usize> {
+        let mut freqs = HashMap::new();
+
+        for c in s.chars() {
+            *freqs.entry(c).or_insert(0) += 1;
+        }
+
+        freqs
+    }
+
+    pub fn frequency_sort(mut s: String) -> String {
+        let mut freqs: Vec<(char, usize)> = Solution::get_freqs(&s).into_iter().collect();
+        freqs.sort_by_key(|(_, count)| cmp::Reverse(*count));
+
+        freqs.into_iter().fold(String::new(), |mut s, (c, n)| {
+            for _ in 0..n {
+                s.push(c);
+            }
+
+            s
+        })
+    }
+}