cs: add «1684. Count the Number of Consistent Strings»

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

cs/count-the-number-of-consistent-strings.cs

@@ -0,0 +1,19 @@
+public class Solution {
+    private static int GetMask(string allowed) {
+        var mask = 0;
+
+        foreach (var c in allowed) {
+            mask |= 1 << (c - 'a');
+        }
+
+        return mask;
+    }
+
+    private static bool IsConsistent(int mask, string word)
+        => word.All(c => (mask & (1 << (c - 'a'))) != 0);
+
+    public int CountConsistentStrings(string allowed, string[] words) {
+        var mask = GetMask(allowed);
+        return words.Count(word => IsConsistent(mask, word));
+    }
+}