rs: add “1207. Unique Number of Occurrences”

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

rs/unique-number-of-occurrences.rs

@@ -0,0 +1,13 @@
+use std::collections::{HashMap, HashSet};
+
+impl Solution {
+    pub fn unique_occurrences(arr: Vec<i32>) -> bool {
+        // count frequencies
+        let mut freqs: HashMap<i32, i32> = HashMap::new();
+        for x in &arr {
+            *freqs.entry(*x).or_insert(0) += 1;
+        }
+
+        freqs.values().collect::<HashSet<_>>().len() == freqs.len()
+    }
+}