problems(rs): add „451. Sort Characters By Frequency“

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

problems/sort-characters-by-frequency.rs

@@ -0,0 +1,18 @@
+use std::cmp::Reverse;
+use std::collections::BTreeMap;
+
+impl Solution {
+    pub fn frequency_sort(s: String) -> String {
+        let mut freqs: BTreeMap<_, usize> = BTreeMap::new();
+
+        s.as_str().chars().for_each(|c| {
+            let current = freqs.get(&c).unwrap_or(&0);
+            freqs.insert(c, 1 + current);
+        });
+
+        let mut frequencies: Vec<_> = freqs.iter().collect();
+        frequencies.sort_by_key(|&(_, count)| Reverse(count));
+
+        frequencies.iter().fold(String::new(), |s, (c, count)| s + &c.to_string().repeat(**count))
+    }
+}