rs: add «791. Custom Sort String»

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

rs/custom-sort-string.rs

@@ -0,0 +1,13 @@
+use std::collections::HashMap;
+
+impl Solution {
+    pub fn custom_sort_string(order: String, s: String) -> String {
+        let weights: HashMap<char, usize> =
+            order.chars().enumerate().map(|(i, c)| (c, i)).collect();
+
+        let mut s: Vec<char> = s.chars().collect();
+        s.sort_unstable_by_key(|c| *weights.get(c).unwrap_or(&usize::MAX));
+
+        s.into_iter().collect()
+    }
+}