rs: add «621. Task Scheduler»

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

rs/task-scheduler.rs

@@ -0,0 +1,21 @@
+use std::cmp;
+use std::collections::HashMap;
+
+impl Solution {
+    pub fn least_interval(tasks: Vec<char>, n: i32) -> i32 {
+        let mut freqs: HashMap<char, i32> = HashMap::new();
+        let mut found_max = 0;
+
+        for c in &tasks {
+            let mut counter = freqs.entry(*c).or_insert(0);
+
+            *counter += 1;
+            found_max = cmp::max(found_max, *counter);
+        }
+
+        let mut time = (1 + n) * (found_max - 1);
+        time += freqs.values().filter(|&&c| c == found_max).count() as i32;
+
+        cmp::max(tasks.len().try_into().unwrap(), time)
+    }
+}