cs: add «1331. Rank Transform of an Array»

Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/rank-transform-of-an-array.cs

@@ -0,0 +1,14 @@
+public class Solution {
+    public int[] ArrayRankTransform(int[] arr) {
+        var unique = new SortedSet<int>();
+        foreach (var x in arr) {
+            unique.Add(x);
+        }
+
+        var ranks = unique.Select((item, index) => (item, index + 1)).ToDictionary();
+        for (int i = 0; i < arr.Length; ++i) {
+            arr[i] = ranks[arr[i]];
+        }
+        return arr;
+    }
+}