rs: add «3005. Count Elements With Maximum Frequency»

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

rs/count-elements-with-maximum-frequency.rs

@@ -0,0 +1,13 @@
+use std::collections::HashMap;
+
+impl Solution {
+    pub fn max_frequency_elements(nums: Vec<i32>) -> i32 {
+        let mut freqs: HashMap<i32, usize> = HashMap::new();
+        for &x in &nums {
+            *freqs.entry(x).or_insert(0) += 1;
+        }
+
+        let m = *freqs.values().max().unwrap();
+        (m * freqs.iter().filter(|&(_, f)| *f == m).count()) as i32
+    }
+}